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.BufferedReader;
  import java.io.IOException;
  import java.io.InputStreamReader;
  import java.io.File;
  import java.io.*;
  import java.util.*;
  
  public class Main {
  
      // static final File ip = new File("input.txt");
      // static final File op = new File("output.txt");
      // static {
      // try {
      // System.setOut(new PrintStream(op));
      // System.setIn(new FileInputStream(ip));
      // } catch (Exception e) {
      // }
      // }
      static long MOD = (long) 1e9 + 7;
  
      public static void main(String[] args) {
          FastReader sc = new FastReader();
          int test = sc.nextInt();
          while(test-- != 0)
          {
              int n = sc.nextInt();
              String s = sc.next();
              String t = s;
              char[] b = t.toCharArray();
              char[] a = s.toCharArray();
              Arrays.sort(a);
              int cnt = 0;
              for(int i=0;i<n;i++)
              {
                 if(a[i]==b[i])
                 continue;
                 else
                 cnt++;
              }
              System.out.println(cnt);
          }
              
      }
  
      static long power(long x, long y, long p) {
          long res = 1;
          x = x % p;
          if (x == 0)
              return 0;
          while (y > 0) {
              if ((y & 1) != 0)
                  res = (res * x) % p;
              y = y >> 1;
              x = (x * x) % p;
          }
          return res;
      }
  
      public static int countSetBits(long number) {
          int count = 0;
          while (number > 0) {
              ++count;
              number &= number - 1;
          }
          return count;
      }
  
      private static <T> void swap(T[] array, int i, int j) {
          if (i != j) {
              T tmp = array[i];
              array[i] = array[j];
              array[j] = tmp;
          }
      }
  
      private static long getSum(int[] array) {
          long sum = 0;
          for (int value : array) {
              sum += value;
          }
          return sum;
      }
  
      private static boolean isPrime(Long x) {
          if (x < 2)
              return false;
          for (long d = 2; d * d <= x; ++d) {
              if (x % d == 0)
                  return false;
          }
          return true;
      }
  
      static int[] reverse(int a[], int n) {
          int i, k, t;
          for (i = 0; i < n / 2; i++) {
              t = a[i];
              a[i] = a[n - i - 1];
              a[n - i - 1] = t;
          }
          return a;
      }
  
      private static boolean isPrimeInt(int x) {
          if (x < 2)
              return false;
          for (int d = 2; d * d <= x; ++d) {
              if (x % d == 0)
                  return false;
          }
          return true;
      }
  
      public static String reverse(String input) {
          StringBuilder str = new StringBuilder("");
  
          for (int i = input.length() - 1; i >= 0; i--) {
              str.append(input.charAt(i));
          }
  
          return str.toString();
      }
  
      private static int[] getPrimes(int n) {
          boolean[] used = new boolean[n + 1];
          used[0] = used[1] = true;
          int size = 0;
          for (int i = 2; i <= n; ++i) {
              if (!used[i]) {
                  ++size;
                  for (int j = 2 * i; j <= n; j += i) {
                      used[j] = true;
                  }
              }
          }
          int[] primes = new int[size];
          for (int i = 0, cur = 0; i <= n; ++i) {
              if (!used[i]) {
                  primes[cur++] = i;
              }
          }
          return primes;
      }
  
      private static long lcm(long a, long b) {
          return a / gcd(a, b) * b;
      }
  
      private static long gcd(long a, long b) {
          return (a == 0 ? b : gcd(b % a, a));
      }
  
      static void shuffleArray(int[] arr) {
          int n = arr.length;
          Random rnd = new Random();
          for (int i = 0; i < n; ++i) {
              int tmp = arr[i];
              int randomPos = i + rnd.nextInt(n - i);
              arr[i] = arr[randomPos];
              arr[randomPos] = tmp;
          }
      }
  
      static void shuffleList(ArrayList<Long> arr) {
          int n = arr.size();
          Random rnd = new Random();
          for (int i = 0; i < n; ++i) {
              long tmp = arr.get(i);
              int randomPos = i + rnd.nextInt(n - i);
              arr.set(i, arr.get(randomPos));
              arr.set(randomPos, tmp);
          }
      }
  
      static void factorize(long n) {
          int count = 0;
          while (!(n % 2 > 0)) {
              n >>= 1;
  
              count++;
          }
          if (count > 0) {
              // System.out.println("2" + " " + count);
          }
          for (long i = 3; i <= (long) Math.sqrt(n); i += 2) {
              count = 0;
              while (n % i == 0) {
                  count++;
                  n = n / i;
              }
              if (count > 0) {
                  // System.out.println(i + " " + count);
              }
          }
  
          if (n > 2) {
              // System.out.println(i + " " + count);
          }
      }
  
      static void shuffleArrayL(long[] arr) {
          int n = arr.length;
          Random rnd = new Random();
          for (int i = 0; i < n; ++i) {
              long tmp = arr[i];
              int randomPos = i + rnd.nextInt(n - i);
              arr[i] = arr[randomPos];
              arr[randomPos] = tmp;
          }
      }
  
      static class FastReader {
          BufferedReader br;
          StringTokenizer st;
  
          public FastReader() {
              br = new BufferedReader(new InputStreamReader(System.in));
          }
  
          public boolean hasNext() {
              return false;
          }
  
          String next() {
              while (st == null || !st.hasMoreElements()) {
                  try {
                      st = new StringTokenizer(br.readLine());
                  } catch (IOException e) {
                      e.printStackTrace();
                  }
              }
              return st.nextToken();
          }
  
          int nextInt() {
              return Integer.parseInt(next());
          }
  
          long nextLong() {
              return Long.parseLong(next());
          }
  
          double nextDouble() {
              return Double.parseDouble(next());
          }
  
          String nextLine() {
              String str = "";
              try {
                  str = br.readLine();
              } catch (IOException e) {
                  e.printStackTrace();
              }
              return str;
          }
      }
  }