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

public class D {
    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 PrintWriter out = new PrintWriter(System.out);

    private static int[] rai(int n) {
        int[] arr = new int[n];
        for (int i = 0; i < n; i++) {
            arr[i] = s.nextInt();
        }
        return arr;
    }

    private static int[][] rai(int n, int m) {
        int[][] arr = new int[n][m];
        for (int i = 0; i < n; i++) {
            for (int j = 0; j < m; j++) {
                arr[i][j] = s.nextInt();
            }
        }
        return arr;
    }

    private static long[] ral(int n) {
        long[] arr = new long[n];
        for (int i = 0; i < n; i++) {
            arr[i] = s.nextLong();
        }
        return arr;
    }

    private static long[][] ral(int n, int m) {
        long[][] arr = new long[n][m];
        for (int i = 0; i < n; i++) {
            for (int j = 0; j < m; j++) {
                arr[i][j] = s.nextLong();
            }
        }
        return arr;
    }

    private static int ri() {
        return s.nextInt();
    }

    private static long rl() {
        return s.nextLong();
    }

    private static String rs() {
        return s.next();
    }

    static int gcd(int a,int b)
    {
        if(b==0)
        {
            return a;
        }
        return gcd(b,a%b);
    }

    static boolean isPrime(int n) {
        //check if n is a multiple of 2
        if (n % 2 == 0) return false;
        //if not, then just check the odds
        for (int i = 3; i <= Math.sqrt(n); i += 2) {
            if (n % i == 0)
                return false;
        }
        return true;
    }


//    static int count=0;
    static int countMoves(String str,char c)
    {
        int n=str.length();
        int count=0;
        if(n==1)
        {
            if(str.charAt(0)!=c)
            {
                return 1;
            }
            return 0;
        }
        int c1=0;
        int c2=0;
        for(int i=0;i<n/2;i++)
        {
            if(str.charAt(i)==c)
            {
                c1++;
            }
        }
        for(int i=n/2;i<n;i++)
        {
            if(str.charAt(i)==c)
            {
                c2++;
            }
        }

        int count1=n/2-c1+countMoves(str.substring(n/2),(char)(c+1));
        int count2=n/2-c2+countMoves(str.substring(0,n/2),(char)(c+1));
        count+=Math.min(count1,count2);
        return count;
    }
    public static void main(String[] args) {
        StringBuilder ans = new StringBuilder();
        int t = ri();
//        int t=1;
        while (t-- > 0) {
            int n=ri();
            String str=rs();
            if(n==1)
            {
                if(str.charAt(0)=='a')
                    ans.append("0\n");
                else ans.append("1\n");
                continue;
            }

            int res=countMoves(str,'a');
            ans.append(res).append("\n");



        }
        out.print(ans.toString());
        out.flush();

    }

}


