description: A string s of length n, consisting of lowercase letters of the English alphabet, is given.
  
  You must choose some number k between 0 and n. Then, you select k characters of s and permute them however you want. In this process, the positions of the other n-k characters remain unchanged. You have to perform this operation exactly once.
  
  For example, if s="andrea", you can choose the k=4 characters "a_d_ea" and permute them into "d_e_aa" so that after the operation the string becomes "dneraa".
  
  Determine the minimum k so that it is possible to sort s alphabetically (that is, after the operation its characters appear in alphabetical order).
  
  Input
  
  The first line contains a single integer t (1 ≤ t ≤ 1000) — the number of test cases. Then t test cases follow.
  
  The first line of each test case contains one integer n (1 ≤ n ≤ 40) — the length of the string.
  
  The second line of each test case contains the string s. It is guaranteed that s contains only lowercase letters of the English alphabet.
  
  Output
  
  For each test case, output the minimum k that allows you to obtain a string sorted alphabetically, through the operation described above.
  
  Example
  
  Input
  
  
  4
  3
  lol
  10
  codeforces
  5
  aaaaa
  4
  dcba
  
  
  Output
  
  
  2
  6
  0
  4
  
  Note
  
  In the first test case, we can choose the k=2 characters "_ol" and rearrange them as "_lo" (so the resulting string is "llo"). It is not possible to sort the string choosing strictly less than 2 characters.
  
  In the second test case, one possible way to sort s is to consider the k=6 characters "_o__force_" and rearrange them as "_c__efoor_" (so the resulting string is "ccdeefoors"). One can show that it is not possible to sort the string choosing strictly less than 6 characters.
  
  In the third test case, string s is already sorted (so we can choose k=0 characters).
  
  In the fourth test case, we can choose all k=4 characters "dcba" and reverse the whole string (so the resulting string is "abcd").
solution: //package contest.CodeforcesGlobalRound15;
  
  import java.io.*;
  import java.util.*;
  
  public class A1552 {
      InputStream is;
      FastWriter  out;
      String      INPUT = "";
  
      //提交时注意需要注释掉首行package
      //基础类型数组例如long[]使用Arrays排序容易TLE,可以替换成Long[]
      //int 最大值2**31-1,2147483647;
      //尽量使用long类型,避免int计算的数据溢出
      void solve() {
          int t = ni();
          for (; t > 0; t--)
              go();
      }
  
      void go() {
          int n = ni();
          String s = ns();
          char[] tmp = s.toCharArray();
          char[] ss = s.toCharArray();
          Arrays.sort(ss);
  //        out.println(ss);
          int ans = 0;
          for (int i = 0; i < n; i++) {
              if (ss[i] != tmp[i]) {
                  ans++;
              }
          }
          out.println(ans);
      }
  
      void run() throws Exception {
          is = System.in;
          out = new FastWriter(System.out);
  
          long s = System.currentTimeMillis();
          solve();
          out.flush();
          //debug log
          //tr(System.currentTimeMillis() - s + "ms");
      }
  
      public static void main(String[] args) throws Exception {
          new A1552().run();
      }
  
      private byte[] inbuf  = new byte[1024];
      public int     lenbuf = 0, ptrbuf = 0;
  
      private int readByte() {
          if (lenbuf == -1)
              throw new InputMismatchException();
          if (ptrbuf >= lenbuf) {
              ptrbuf = 0;
              try {
                  lenbuf = is.read(inbuf);
              } catch (IOException e) {
                  throw new InputMismatchException();
              }
              if (lenbuf <= 0)
                  return -1;
          }
          return inbuf[ptrbuf++];
      }
  
      private boolean isSpaceChar(int c) {
          return !(c >= 33 && c <= 126);
      }
  
      private int skip() {
          int b;
          while ((b = readByte()) != -1 && isSpaceChar(b))
              ;
          return b;
      }
  
      private double nd() {
          return Double.parseDouble(ns());
      }
  
      private char nc() {
          return (char) skip();
      }
  
      private String ns() {
          int b = skip();
          StringBuilder sb = new StringBuilder();
          while (!(isSpaceChar(b))) { // when nextLine, (isSpaceChar(b) && b != ' ')
              sb.appendCodePoint(b);
              b = readByte();
          }
          return sb.toString();
      }
  
      private char[] ns(int n) {
          char[] buf = new char[n];
          int b = skip(), p = 0;
          while (p < n && !(isSpaceChar(b))) {
              buf[p++] = (char) b;
              b = readByte();
          }
          return n == p ? buf : Arrays.copyOf(buf, p);
      }
  
      private Integer[] na(int n) {
          Integer[] a = new Integer[n];
          for (int i = 0; i < n; i++)
              a[i] = ni();
          return a;
      }
  
      private Long[] nal(int n) {
          Long[] a = new Long[n];
          for (int i = 0; i < n; i++)
              a[i] = nl();
          return a;
      }
  
      private char[][] nm(int n, int m) {
          char[][] map = new char[n][];
          for (int i = 0; i < n; i++)
              map[i] = ns(m);
          return map;
      }
  
      private Integer[][] nmi(int n, int m) {
          Integer[][] map = new Integer[n][];
          for (int i = 0; i < n; i++)
              map[i] = na(m);
          return map;
      }
  
      private int ni() {
          return (int) nl();
      }
  
      private long nl() {
          long num = 0;
          int b;
          boolean minus = false;
          while ((b = readByte()) != -1 && !((b >= '0' && b <= '9') || b == '-'))
              ;
          if (b == '-') {
              minus = true;
              b = readByte();
          }
  
          while (true) {
              if (b >= '0' && b <= '9') {
                  num = num * 10 + (b - '0');
              } else {
                  return minus ? -num : num;
              }
              b = readByte();
          }
      }
  
      public static class FastWriter {
          private static final int   BUF_SIZE = 1 << 13;
          private final byte[]       buf      = new byte[BUF_SIZE];
          private final OutputStream out;
          private int                ptr      = 0;
  
          private FastWriter() {
              out = null;
          }
  
          public FastWriter(OutputStream os) {
              this.out = os;
          }
  
          public FastWriter(String path) {
              try {
                  this.out = new FileOutputStream(path);
              } catch (FileNotFoundException e) {
                  throw new RuntimeException("FastWriter");
              }
          }
  
          public FastWriter write(byte b) {
              buf[ptr++] = b;
              if (ptr == BUF_SIZE)
                  innerflush();
              return this;
          }
  
          public FastWriter write(char c) {
              return write((byte) c);
          }
  
          public FastWriter write(char[] s) {
              for (char c : s) {
                  buf[ptr++] = (byte) c;
                  if (ptr == BUF_SIZE)
                      innerflush();
              }
              return this;
          }
  
          public FastWriter write(String s) {
              s.chars().forEach(c -> {
                  buf[ptr++] = (byte) c;
                  if (ptr == BUF_SIZE)
                      innerflush();
              });
              return this;
          }
  
          private static int countDigits(int l) {
              if (l >= 1000000000)
                  return 10;
              if (l >= 100000000)
                  return 9;
              if (l >= 10000000)
                  return 8;
              if (l >= 1000000)
                  return 7;
              if (l >= 100000)
                  return 6;
              if (l >= 10000)
                  return 5;
              if (l >= 1000)
                  return 4;
              if (l >= 100)
                  return 3;
              if (l >= 10)
                  return 2;
              return 1;
          }
  
          public FastWriter write(int x) {
              if (x == Integer.MIN_VALUE) {
                  return write((long) x);
              }
              if (ptr + 12 >= BUF_SIZE)
                  innerflush();
              if (x < 0) {
                  write((byte) '-');
                  x = -x;
              }
              int d = countDigits(x);
              for (int i = ptr + d - 1; i >= ptr; i--) {
                  buf[i] = (byte) ('0' + x % 10);
                  x /= 10;
              }
              ptr += d;
              return this;
          }
  
          private static int countDigits(long l) {
              if (l >= 1000000000000000000L)
                  return 19;
              if (l >= 100000000000000000L)
                  return 18;
              if (l >= 10000000000000000L)
                  return 17;
              if (l >= 1000000000000000L)
                  return 16;
              if (l >= 100000000000000L)
                  return 15;
              if (l >= 10000000000000L)
                  return 14;
              if (l >= 1000000000000L)
                  return 13;
              if (l >= 100000000000L)
                  return 12;
              if (l >= 10000000000L)
                  return 11;
              if (l >= 1000000000L)
                  return 10;
              if (l >= 100000000L)
                  return 9;
              if (l >= 10000000L)
                  return 8;
              if (l >= 1000000L)
                  return 7;
              if (l >= 100000L)
                  return 6;
              if (l >= 10000L)
                  return 5;
              if (l >= 1000L)
                  return 4;
              if (l >= 100L)
                  return 3;
              if (l >= 10L)
                  return 2;
              return 1;
          }
  
          public FastWriter write(long x) {
              if (x == Long.MIN_VALUE) {
                  return write("" + x);
              }
              if (ptr + 21 >= BUF_SIZE)
                  innerflush();
              if (x < 0) {
                  write((byte) '-');
                  x = -x;
              }
              int d = countDigits(x);
              for (int i = ptr + d - 1; i >= ptr; i--) {
                  buf[i] = (byte) ('0' + x % 10);
                  x /= 10;
              }
              ptr += d;
              return this;
          }
  
          public FastWriter write(double x, int precision) {
              if (x < 0) {
                  write('-');
                  x = -x;
              }
              x += Math.pow(10, -precision) / 2;
              //		if(x < 0){ x = 0; }
              write((long) x).write(".");
              x -= (long) x;
              for (int i = 0; i < precision; i++) {
                  x *= 10;
                  write((char) ('0' + (int) x));
                  x -= (int) x;
              }
              return this;
          }
  
          public FastWriter writeln(char c) {
              return write(c).writeln();
          }
  
          public FastWriter writeln(int x) {
              return write(x).writeln();
          }
  
          public FastWriter writeln(long x) {
              return write(x).writeln();
          }
  
          public FastWriter writeln(double x, int precision) {
              return write(x, precision).writeln();
          }
  
          public FastWriter write(int... xs) {
              boolean first = true;
              for (int x : xs) {
                  if (!first)
                      write(' ');
                  first = false;
                  write(x);
              }
              return this;
          }
  
          public FastWriter write(long... xs) {
              boolean first = true;
              for (long x : xs) {
                  if (!first)
                      write(' ');
                  first = false;
                  write(x);
              }
              return this;
          }
  
          public FastWriter writeln() {
              return write((byte) '\n');
          }
  
          public FastWriter writeln(int... xs) {
              return write(xs).writeln();
          }
  
          public FastWriter writeln(long... xs) {
              return write(xs).writeln();
          }
  
          public FastWriter writeln(char[] line) {
              return write(line).writeln();
          }
  
          public FastWriter writeln(char[]... map) {
              for (char[] line : map)
                  write(line).writeln();
              return this;
          }
  
          public FastWriter writeln(String s) {
              return write(s).writeln();
          }
  
          private void innerflush() {
              try {
                  out.write(buf, 0, ptr);
                  ptr = 0;
              } catch (IOException e) {
                  throw new RuntimeException("innerflush");
              }
          }
  
          public void flush() {
              innerflush();
              try {
                  out.flush();
              } catch (IOException e) {
                  throw new RuntimeException("flush");
              }
          }
  
          public FastWriter print(byte b) {
              return write(b);
          }
  
          public FastWriter print(char c) {
              return write(c);
          }
  
          public FastWriter print(char[] s) {
              return write(s);
          }
  
          public FastWriter print(String s) {
              return write(s);
          }
  
          public FastWriter print(int x) {
              return write(x);
          }
  
          public FastWriter print(long x) {
              return write(x);
          }
  
          public FastWriter print(double x, int precision) {
              return write(x, precision);
          }
  
          public FastWriter println(char c) {
              return writeln(c);
          }
  
          public FastWriter println(int x) {
              return writeln(x);
          }
  
          public FastWriter println(long x) {
              return writeln(x);
          }
  
          public FastWriter println(double x, int precision) {
              return writeln(x, precision);
          }
  
          public FastWriter print(int... xs) {
              return write(xs);
          }
  
          public FastWriter print(long... xs) {
              return write(xs);
          }
  
          public FastWriter println(int... xs) {
              return writeln(xs);
          }
  
          public FastWriter println(long... xs) {
              return writeln(xs);
          }
  
          public FastWriter println(char[] line) {
              return writeln(line);
          }
  
          public FastWriter println(char[]... map) {
              return writeln(map);
          }
  
          public FastWriter println(String s) {
              return writeln(s);
          }
  
          public FastWriter println() {
              return writeln();
          }
      }
  
      public void trnz(int... o) {
          for (int i = 0; i < o.length; i++)
              if (o[i] != 0)
                  System.out.print(i + ":" + o[i] + " ");
          System.out.println();
      }
  
      // print ids which are 1
      public void trt(long... o) {
          Queue<Integer> stands = new ArrayDeque<>();
          for (int i = 0; i < o.length; i++) {
              for (long x = o[i]; x != 0; x &= x - 1)
                  stands.add(i << 6 | Long.numberOfTrailingZeros(x));
          }
          System.out.println(stands);
      }
  
      public void tf(boolean... r) {
          for (boolean x : r)
              System.out.print(x ? '#' : '.');
          System.out.println();
      }
  
      public void tf(boolean[]... b) {
          for (boolean[] r : b) {
              for (boolean x : r)
                  System.out.print(x ? '#' : '.');
              System.out.println();
          }
          System.out.println();
      }
  
      public void tf(long[]... b) {
          if (INPUT.length() != 0) {
              for (long[] r : b) {
                  for (long x : r) {
                      for (int i = 0; i < 64; i++) {
                          System.out.print(x << ~i < 0 ? '#' : '.');
                      }
                  }
                  System.out.println();
              }
              System.out.println();
          }
      }
  
      public void tf(long... b) {
          if (INPUT.length() != 0) {
              for (long x : b) {
                  for (int i = 0; i < 64; i++) {
                      System.out.print(x << ~i < 0 ? '#' : '.');
                  }
              }
              System.out.println();
          }
      }
  
      private void tr(Object... o) {
          System.out.println(Arrays.deepToString(o));
      }
  }
  