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 com.company;
  
  
  
  import  java.math.*;
  import java.util.*;
  import java.lang.*;
  import java.io.*;
  
  
  
  public final class Main {
  
      FastReader s;
  
  
      public static void main (String[] args) throws java.lang.Exception
      {
  
          new Main().run();
  
      }
      void run()
      {
          s = new FastReader();
          solve();
      }
     StringBuffer sb;
     // int counter;
      void solve()
      {
           sb = new StringBuffer();
  
          for(int T = s.nextInt();T > 0;T--)
          {
  
  
              start();
  
  
          }
  
         // System.out.print(sb);
  
      }
  
  
      void start()
      {
          int n = s.nextInt();
  
         String x = s.nextLine();
         String g = sortString(x);
         // System.out.println(x+" "+g);
         int k = 0;
         for(int i = 0; i<x.length(); i++)
         {
                if(x.charAt(i) != g.charAt(i))
                    k++;
         }
          System.out.println(k);
  
      }
      public static String sortString(String inputString)
      {
          // convert input string to char array
          char tempArray[] = inputString.toCharArray();
  
          // sort tempArray
          Arrays.sort(tempArray);
  
          // return new sorted string
          return new String(tempArray);
      }
  
  
  
  
  
      long gcd(long a, long b)
      {
          if (b == 0)
              return a;
          return gcd(b, a % b);
      }
      long power(long x, long y, long p)
      {
          long res = 1; // Initialize result
  
          // Update x if it is more
          // than or equal to p
          x = x % p;
  
          while (y > 0)
          {
              // If y is odd, multiply
              // x with the result
              if ((y & 1) > 0)
                  res = (res * x) % p;
  
              // y must be even now
              y = y >> 1; // y = y/2
              x = (x * x) % p;
          }
          return res;
      }
  
      int lower_bound(int [] arr , int key)
      {
  
          int i = 0;
          int j = arr.length-1;
            //if(arr[i] > key)return -1;
           if(arr[j] < key)return -1;
  
          while(i<j)
          {
              int mid = (i+j)/2;
  
              if(arr[mid] == key)
              {
                  j = mid;
              }
  
              else if(arr[mid] < key)
              {
                  i = mid+1;
              }
              else
                  j = mid-1;
        }
  
          return i;
      }
  
      int upper_bound(int [] arr , int key)
      {
  
          int i = 0;
          int j = arr.length-1;
          if(arr[j] <= key)return j+1;
  
          while(i<j)
          {
              int mid = (i+j)/2;
  
  
  
               if(arr[mid] <= key)
              {
                  i = mid+1;
              }
              else
                  j = mid;
          }
  
          return i;
      }
  
  
      static void sort(int[] A){
          int n = A.length;
          Random rnd = new Random();
          for(int i=0; i<n; ++i){
              int tmp = A[i];
              int randomPos = i + rnd.nextInt(n-i);
              A[i] = A[randomPos];
              A[randomPos] = tmp;
          }
          Arrays.sort(A);
      }
      static void sort(long[] A){
          int n = A.length;
          Random rnd = new Random();
          for(int i=0; i<n; ++i){
              long tmp = A[i];
              int randomPos = i + rnd.nextInt(n-i);
              A[i] = A[randomPos];
              A[randomPos] = tmp;
          }
          Arrays.sort(A);
      }
      static String sort(String s) {
          Character ch[]=new Character[s.length()];
          for(int i=0;i<s.length();i++) {
              ch[i]=s.charAt(i);
          }
          Arrays.sort(ch);
          StringBuffer st=new StringBuffer("");
          for(int i=0;i<s.length();i++) {
              st.append(ch[i]);
          }
          return st.toString();
      }
  
      //long array input
      public long [] longArr(int len)
      {
          // long arr input
          long [] arr = new long[len];
          String [] strs =  s.nextLine().trim().split("\\s+");
          for(int i =0; i<len; i++)
          {
              arr[i] = Long.parseLong(strs[i]);
  
          }
          return arr;
      }
      // int arr input
      public int [] intArr(int len)
      {
          // long arr input
          int [] arr = new int[len];
          String [] strs =  s.nextLine().trim().split("\\s+");
          for(int i =0; i<len; i++)
          {
              arr[i] = Integer.parseInt(strs[i]);
  
          }
          return arr;
      }
  
      static class FastReader
      {
          BufferedReader br;
          StringTokenizer st;
  
          public FastReader()
          {
              br = new BufferedReader(new
                      InputStreamReader(System.in));
          }
  
          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;
          }
      }
  
  
  
  }
  