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 Codeforces.GR15;
  
  import java.io.BufferedReader;
  import java.io.IOException;
  import java.io.InputStreamReader;
  import java.io.PrintWriter;
  import java.util.Arrays;
  import java.util.StringTokenizer;
  
  public class A {
  
      static final long mod = (long) 1e9 + 7l;
  
      private static void solve(int t) {
          int n = fs.nextInt();
          char[] chr = fs.next().toCharArray();
          char[] sorted = new char[n];
          for (int i = 0; i < n; i++) {
              sorted[i] = chr[i];
          }
          Arrays.sort(sorted);
          int k = 0;
          for (int i = 0; i < n; i++) {
              if (chr[i] != sorted[i]) k++;
          }
          System.out.println(k);
      }
  
      static class pair {
          int a;
          int b;
  
          public pair(int a, int b) {
              this.a = a;
              this.b = b;
          }
  
          @Override
          public String toString() {
              return "pair{" +
                      "a=" + a +
                      ", b=" + b +
                      '}';
          }
      }
  
      public static void main(String[] args) {
          fs = new FastScanner();
          out = new PrintWriter(System.out);
          long s = System.currentTimeMillis();
          int t = fs.nextInt();
          for (int i = 1; i <= t; i++) solve(t);
          out.close();
  //        System.err.println( System.currentTimeMillis() - s + "ms" );
      }
  
      static boolean DEBUG = true;
      static PrintWriter out;
      static FastScanner fs;
  
      static void trace(Object... o) {
          if (!DEBUG) return;
          System.err.println(Arrays.deepToString(o));
      }
  
      static void pl(Object o) {
          out.println(o);
      }
  
      static void p(Object o) {
          out.print(o);
      }
  
      static long gcd(long a, long b) {
          return (b == 0) ? a : gcd(b, a % b);
      }
  
      static int gcd(int a, int b) {
          return (b == 0) ? a : gcd(b, a % b);
      }
  
      static void sieveOfEratosthenes(int n, int factors[]) {
          factors[1] = 1;
          for (int p = 2; p * p <= n; p++) {
              if (factors[p] == 0) {
                  factors[p] = p;
                  for (int i = p * p; i <= n; i += p)
                      factors[i] = p;
              }
          }
      }
  
      static long mul(long a, long b) {
          return a * b % mod;
      }
  
      static long fact(int x) {
          long ans = 1;
          for (int i = 2; i <= x; i++) ans = mul(ans, i);
          return ans;
      }
  
      static long fastPow(long base, long exp) {
          if (exp == 0) return 1;
          long half = fastPow(base, exp / 2);
          if (exp % 2 == 0) return mul(half, half);
          return mul(half, mul(half, base));
      }
  
      static long modInv(long x) {
          return fastPow(x, mod - 2);
      }
  
      static long nCk(int n, int k) {
          return mul(fact(n), mul(modInv(fact(k)), modInv(fact(n - k))));
      }
  
      static class FastScanner {
          BufferedReader br = new BufferedReader(new InputStreamReader(System.in));
          StringTokenizer st = new StringTokenizer("");
  
          public String next() {
              while (!st.hasMoreElements())
                  try {
                      st = new StringTokenizer(br.readLine());
                  } catch (IOException e) {
                      e.printStackTrace();
                  }
              return st.nextToken();
          }
  
          int nextInt() {
              return Integer.parseInt(next());
          }
  
          int[] readArray(int n) {
              int[] a = new int[n];
              for (int i = 0; i < n; i++)
                  a[i] = nextInt();
              return a;
          }
  
          long nextLong() {
              return Long.parseLong(next());
          }
      }
  }
  