import java.util.*;
import java.lang.*;
import java.io.*;

public final class Solution{
   static int solve(int[] arr, int curr, int l, int r){
       if(curr>26){
           return 0;
       }
       if(l==r){
      
           if(arr[l]==curr){
               return 0;
           }
           return 1;
       }
      int mid=(l+r)/2;
      int c1=0;
      int c2=0;
      for(int i=l;i<=mid;i++){
          if(arr[i]==curr){
              c1++;
          }
      }
      for(int i=mid+1;i<=r;i++){
          if(arr[i]==curr){
              c2++;
          }
      }
    
      int ans=0;
      
         
          ans=(r-mid)-c2;
  
        int val1= ans+solve(arr,curr+1,l,mid);
     
          
        ans=(r-mid)-c1;
         int val2 = ans+solve(arr,curr+1,mid+1,r);
      return Math.min(val1,val2);

   }
  
     
   public static void main (String[] args) throws Exception
   {  
    
      BufferedWriter op = new BufferedWriter(new OutputStreamWriter(System.out));
       Scanner sc = new Scanner(System.in);
       int t=sc.nextInt();
           while(t-->0){
             int n=sc.nextInt();
             String s=sc.next();
             int[] arr= new int[n];
             for(int i=0;i<n;i++){
                 arr[i]=(int)(s.charAt(i)-'a');
             }
            //  debug.print1d(arr);
            int ans=solve(arr,0,0,n-1); 
            op.write(ans+"\n");
            
          
           }
           op.flush();
  
       
        
        
          
          
         
    }



     
           
           
           

            
         

           
          


     


   


 
 
 
 }





 

 class debug{
         
    
   static void print1d(long [] arr){
       System.out.println();
       for(int i=0;i<arr.length;i++){
           System.out.print(arr[i]+" ");
       }
       System.out.println();
   }
     static void print1d(int [] arr){
       System.out.println();
       for(int i=0;i<arr.length;i++){
           System.out.print(arr[i]+" ");
       }
       System.out.println();
   }
   
    static void print2d(int[][] arr){
        System.out.println();
        int n=arr.length;
        int n2=arr[0].length;
        for(int i=0;i<n;i++){
            for(int j=0;j<n2;j++){
                System.out.print(arr[i][j]+" ");
            }
            System.out.println();
        }
        System.out.println();
    }
     static void print2d(long[][] arr){
        System.out.println();
        int n=arr.length;
        int n2=arr[0].length;
        for(int i=0;i<n;i++){
            for(int j=0;j<n2;j++){
                System.out.print(arr[i][j]+" ");
            }
            System.out.println();
        }
        System.out.println();
    }



 }
  


  class pair{
      long no;
      int ind;
      pair(long no, int ind){
          this.no=no;
          this.ind=ind;
      }
  }
  class comp implements Comparator<pair>{
      public int compare(pair p1, pair p2){
          if(p1.no>p2.no){
              return 1;
          }
          return -1;
      }
  }

 
 class Reader {
        final private int BUFFER_SIZE = 1 << 16;
        private DataInputStream din;
        private byte[] buffer;
        private int bufferPointer, bytesRead;
 
        public Reader()
        {
            din = new DataInputStream(System.in);
            buffer = new byte[BUFFER_SIZE];
            bufferPointer = bytesRead = 0;
        }
 
        public Reader(String file_name) throws IOException
        {
            din = new DataInputStream(
                new FileInputStream(file_name));
            buffer = new byte[BUFFER_SIZE];
            bufferPointer = bytesRead = 0;
        }
 
        public String readLine() throws IOException
        {
            byte[] buf = new byte[64]; // line length
            int cnt = 0, c;
            while ((c = read()) != -1) {
                if (c == '\n') {
                    if (cnt != 0) {
                        break;
                    }
                    else {
                        continue;
                    }
                }
                buf[cnt++] = (byte)c;
            }
            return new String(buf, 0, cnt);
        }
 
        public int nextInt() throws IOException
        {
            int ret = 0;
            byte c = read();
            while (c <= ' ') {
                c = read();
            }
            boolean neg = (c == '-');
            if (neg)
                c = read();
            do {
                ret = ret * 10 + c - '0';
            } while ((c = read()) >= '0' && c <= '9');
 
            if (neg)
                return -ret;
            return ret;
        }
 
        public long nextLong() throws IOException
        {
            long ret = 0;
            byte c = read();
            while (c <= ' ')
                c = read();
            boolean neg = (c == '-');
            if (neg)
                c = read();
            do {
                ret = ret * 10 + c - '0';
            } while ((c = read()) >= '0' && c <= '9');
            if (neg)
                return -ret;
            return ret;
        }
 
        public double nextDouble() throws IOException
        {
            double ret = 0, div = 1;
            byte c = read();
            while (c <= ' ')
                c = read();
            boolean neg = (c == '-');
            if (neg)
                c = read();
 
            do {
                ret = ret * 10 + c - '0';
            } while ((c = read()) >= '0' && c <= '9');
 
            if (c == '.') {
                while ((c = read()) >= '0' && c <= '9') {
                    ret += (c - '0') / (div *= 10);
                }
            }
 
            if (neg)
                return -ret;
            return ret;
        }
 
        private void fillBuffer() throws IOException
        {
            bytesRead = din.read(buffer, bufferPointer = 0,
                                 BUFFER_SIZE);
            if (bytesRead == -1)
                buffer[0] = -1;
        }
 
        private byte read() throws IOException
        {
            if (bufferPointer == bytesRead)
                fillBuffer();
            return buffer[bufferPointer++];
        }
 
        public void close() throws IOException
        {
            if (din == null)
                return;
            din.close();
        }
    }

    

//   BufferedWriter op = new BufferedWriter(new OutputStreamWriter(System.out));


// in comparator to sort in in order return -1 
// in comparator to sort in increasing order return 1


// sorting in increasing 
// class Comp implements Comparator<pair>{
//     public int compare(pair a,pair b){
//         if(a.b>b.b){
//             return 1;    here
//         }
//         return -1;
//     }
// }  



