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: import java.io.OutputStream;
  import java.io.IOException;
  import java.io.InputStream;
  import java.io.OutputStream;
  import java.util.Arrays;
  import java.io.IOException;
  import java.util.Random;
  import java.lang.reflect.Field;
  import java.nio.charset.StandardCharsets;
  import java.io.UncheckedIOException;
  import java.io.Closeable;
  import java.io.InputStream;
  
  /**
   * Built using CHelper plug-in
   * Actual solution is at the top
   */
  public class Main {
      public static void main(String[] args) throws Exception {
          Thread thread = new Thread(null, new TaskAdapter(), "", 1 << 29);
          thread.start();
          thread.join();
      }
  
      static class TaskAdapter implements Runnable {
          @Override
          public void run() {
              InputStream inputStream = System.in;
              OutputStream outputStream = System.out;
              FastInput in = new FastInput(inputStream);
              FastOutput out = new FastOutput(outputStream);
              ASubsequencePermutation solver = new ASubsequencePermutation();
              int testCount = Integer.parseInt(in.next());
              for (int i = 1; i <= testCount; i++)
                  solver.solve(i, in, out);
              out.close();
          }
      }
  
      static class ASubsequencePermutation {
          char[] s = new char[(int) 1e5];
          char[] clone = new char[(int) 1e5];
  
          public void solve(int testNumber, FastInput in, FastOutput out) {
              int n = in.ri();
              in.rs(s);
              for (int i = 0; i < n; i++) {
                  clone[i] = s[i];
              }
              Randomized.shuffle(clone, 0, n);
              Arrays.sort(clone, 0, n);
              int ans = 0;
              for (int i = 0; i < n; i++) {
                  if (clone[i] != s[i]) {
                      ans++;
                  }
              }
              out.println(ans);
          }
  
      }
  
      static class RandomWrapper {
          private Random random;
          public static final RandomWrapper INSTANCE = new RandomWrapper();
  
          public RandomWrapper() {
              this(new Random());
          }
  
          public RandomWrapper(Random random) {
              this.random = random;
          }
  
          public RandomWrapper(long seed) {
              this(new Random(seed));
          }
  
          public int nextInt(int l, int r) {
              return random.nextInt(r - l + 1) + l;
          }
  
      }
  
      static class FastInput {
          private final InputStream is;
          private StringBuilder defaultStringBuf = new StringBuilder(1 << 13);
          private byte[] buf = new byte[1 << 13];
          private int bufLen;
          private int bufOffset;
          private int next;
  
          public FastInput(InputStream is) {
              this.is = is;
          }
  
          private int read() {
              while (bufLen == bufOffset) {
                  bufOffset = 0;
                  try {
                      bufLen = is.read(buf);
                  } catch (IOException e) {
                      bufLen = -1;
                  }
                  if (bufLen == -1) {
                      return -1;
                  }
              }
              return buf[bufOffset++];
          }
  
          public void skipBlank() {
              while (next >= 0 && next <= 32) {
                  next = read();
              }
          }
  
          public String next() {
              return readString();
          }
  
          public int ri() {
              return readInt();
          }
  
          public int readInt() {
              boolean rev = false;
  
              skipBlank();
              if (next == '+' || next == '-') {
                  rev = next == '-';
                  next = read();
              }
  
              int val = 0;
              while (next >= '0' && next <= '9') {
                  val = val * 10 - next + '0';
                  next = read();
              }
  
              return rev ? val : -val;
          }
  
          public String readString(StringBuilder builder) {
              skipBlank();
  
              while (next > 32) {
                  builder.append((char) next);
                  next = read();
              }
  
              return builder.toString();
          }
  
          public String readString() {
              defaultStringBuf.setLength(0);
              return readString(defaultStringBuf);
          }
  
          public int rs(char[] data, int offset) {
              return readString(data, offset);
          }
  
          public int rs(char[] data) {
              return rs(data, 0);
          }
  
          public int readString(char[] data, int offset) {
              skipBlank();
  
              int originalOffset = offset;
              while (next > 32) {
                  data[offset++] = (char) next;
                  next = read();
              }
  
              return offset - originalOffset;
          }
  
      }
  
      static class FastOutput implements AutoCloseable, Closeable, Appendable {
          private static final int THRESHOLD = 32 << 10;
          private OutputStream writer;
          private StringBuilder cache = new StringBuilder(THRESHOLD * 2);
          private static Field stringBuilderValueField;
          private char[] charBuf = new char[THRESHOLD * 2];
          private byte[] byteBuf = new byte[THRESHOLD * 2];
  
          static {
              try {
                  stringBuilderValueField = StringBuilder.class.getSuperclass().getDeclaredField("value");
                  stringBuilderValueField.setAccessible(true);
              } catch (Exception e) {
                  stringBuilderValueField = null;
              }
              stringBuilderValueField = null;
          }
  
          public FastOutput append(CharSequence csq) {
              cache.append(csq);
              return this;
          }
  
          public FastOutput append(CharSequence csq, int start, int end) {
              cache.append(csq, start, end);
              return this;
          }
  
          private void afterWrite() {
              if (cache.length() < THRESHOLD) {
                  return;
              }
              flush();
          }
  
          public FastOutput(OutputStream writer) {
              this.writer = writer;
          }
  
          public FastOutput append(char c) {
              cache.append(c);
              afterWrite();
              return this;
          }
  
          public FastOutput append(int c) {
              cache.append(c);
              afterWrite();
              return this;
          }
  
          public FastOutput println(int c) {
              return append(c).println();
          }
  
          public FastOutput println() {
              return append('\n');
          }
  
          public FastOutput flush() {
              try {
                  if (stringBuilderValueField != null) {
                      try {
                          byte[] value = (byte[]) stringBuilderValueField.get(cache);
                          writer.write(value, 0, cache.length());
                      } catch (Exception e) {
                          stringBuilderValueField = null;
                      }
                  }
                  if (stringBuilderValueField == null) {
                      int n = cache.length();
                      if (n > byteBuf.length) {
                          //slow
                          writer.write(cache.toString().getBytes(StandardCharsets.UTF_8));
  //                writer.append(cache);
                      } else {
                          cache.getChars(0, n, charBuf, 0);
                          for (int i = 0; i < n; i++) {
                              byteBuf[i] = (byte) charBuf[i];
                          }
                          writer.write(byteBuf, 0, n);
                      }
                  }
                  writer.flush();
                  cache.setLength(0);
              } catch (IOException e) {
                  throw new UncheckedIOException(e);
              }
              return this;
          }
  
          public void close() {
              flush();
              try {
                  writer.close();
              } catch (IOException e) {
                  throw new UncheckedIOException(e);
              }
          }
  
          public String toString() {
              return cache.toString();
          }
  
      }
  
      static class Randomized {
          public static void shuffle(char[] data, int from, int to) {
              to--;
              for (int i = from; i <= to; i++) {
                  int s = nextInt(i, to);
                  char tmp = data[i];
                  data[i] = data[s];
                  data[s] = tmp;
              }
          }
  
          public static int nextInt(int l, int r) {
              return RandomWrapper.INSTANCE.nextInt(l, r);
          }
  
      }
  }
  
  