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.util.*;
  public class vamsi
  { 
  	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; 
  		} 
      }
      static FastReader s = new FastReader();
      static StringBuilder sb = new StringBuilder();
  	public static void main(String[] args) 
  	{ 
  		int t=s.nextInt();
  		while(t-->0)
  		{
  		    int n=s.nextInt(),count=0;
  		    String st=s.next();
  		    char c[] = st.toCharArray();
  		    Arrays.sort(c);
  		    for(int i=0;i<n;i++)
  		    {
  		        if(c[i]!=st.charAt(i))
  		        count++;
  		    }
  		    sb.append(count);
  		    sb.append("\n");
  		}
  		System.out.print(sb);
      }
  }
  