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.PrintWriter;
  import java.math.BigInteger;
  import java.text.SimpleDateFormat;
  import java.time.Instant;
  import java.time.LocalDate;
  import java.time.LocalDateTime;
  import java.time.LocalTime;
  import java.util.*;
  import static java.lang.Math.pow;
  import static java.lang.Math.log;
  import static java.lang.Math.ceil;
  import static java.lang.Math.sqrt;
  import static java.lang.Math.max;
  import static java.lang.Math.min;
  import static java.lang.Math.abs;
  import static java.lang.System.out;
  import java.math.BigInteger;
  import java.util.stream.Collectors;
  
  
  public class Main {
  
  	static FastReader sc;
      static PrintWriter out;
      
  	
  	 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 void ruffleSort(long[] a) {
  			int n=a.length;
  			Random r=new Random();
  			for (int i=0; i<a.length; i++) {
  				long oi=r.nextInt(n), temp=a[i];
  				a[i]=a[(int)oi];
  				a[(int)oi]=temp;
  			}
  			Arrays.sort(a);
  		}
  	static void ruffleSort(int[] a) {
  			int n=a.length;
  			Random r=new Random();
  			for (int i=0; i<a.length; i++) {
  				int oi=r.nextInt(n), temp=a[i];
  				a[i]=a[oi];
  				a[oi]=temp;
  			}
  			Arrays.sort(a);
  		}
  	 
  	 
  	 static int gcd(int a, int b)
  	    {
  	      if (b == 0)
  	        return a;
  	      return gcd(b, a % b);
  	    }
  	 
  	 static int lowerBound(int[] a, int low, int high, int element) {
  		    while (low < high) {
  		        int middle = low + (high - low) / 2;
  		        if (element > a[middle]) {
  		            low = middle + 1;
  		        } else {
  		            high = middle;
  		        }
  		    }
  		    return low;
  		}
  
  		static int upperBound(int[] a, int low, int high, int element) {
  		    while (low < high) {
  		        int middle = low + (high - low) / 2;
  		        if (a[middle] > element) {
  		            high = middle;
  		        } else {
  		            low = middle + 1;
  		        }
  		    }
  		    return low;
  		}
  	 
  	 static class Pair {
  		    long x;
  		    long y;
  		  
  		   
  		public Pair(long x, long y)
  		    {
  		        this.x = x;
  		        this.y = y;
  		    }
  		}
  	 public void solve() {
  			int t=sc.nextInt();
  			while(t-->0) {
  				cal();
  		    }
  		 }
  	 
  	 public static String sortString(String inputString)
  	    {
  	        
  	        char tempArray[] = inputString.toCharArray();
  	          
  	        
  	        Arrays.sort(tempArray);
  	          
  	       
  	        return new String(tempArray);
  	    }
  	 
  	 static void cal() {
  		  int n=sc.nextInt();
  		  String s=sc.nextLine();
  		  String t=s;
  		  
  		  String sorted= sortString(t);
  		  
  		  int c=0;
  		  
  	//	  System.out.println(s+" "+sorted);
  		  
  		  for(int i=0;i<s.length();i++) {
  			  if(s.charAt(i)!=sorted.charAt(i)) {
  				  c++;
  			  }
  		  }
  		  
  		  System.out.println(c);
  	 }
  	 
  	
  	
  	public static void main(String[] args) throws IOException {
  		sc = new FastReader();
  	    out = new PrintWriter(System.out);
  	    new Main().solve();
  	    out.flush();
  	}
  
  }
  