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

public class codeforces {

    public static void main(String[] args) {
        InputReader sc = new InputReader(System.in);
        int T=sc.nextInt();
        for (int t = 0; t < T; t++) {
            int n=sc.nextInt();
            String str=sc.nextLine();
            recur('a',str);
            out.println(total);
            total=0;
        }

        out.close();
    }
    static int total=0;

    public static void recur(char ch,String str) {
        int s=0;
        int e=str.length();
        int fh=0;
        int sh=0;
//        System.out.println(s+" "+e+" "+ch+" "+" "+str+" "+total);
        if (s==e-1) {
            total+=str.charAt(s)==ch?0:1;
            return;
        }
        for (int i = s; i < e/2; i++) {
            if (str.charAt(i)!=ch)
                fh++;
        }
        for (int i = e-1; i >= e/2; i--) {
            if (str.charAt(i)!=ch)
                sh++;
        }
        total+=Math.min(fh,sh);
        if(Math.min(fh,sh)==fh) {
            recur(++ch,str.substring(e/2,e));
        }
        else {
            recur(++ch,str.substring(s,e/2));
        }
    }
	
	static class InputReader {

            private final InputStream stream;
            private final byte[] buf = new byte[8192];
            private int curChar, snumChars;
            private SpaceCharFilter filter;

            public InputReader(InputStream stream) {
                this.stream = stream;
            }

            public int snext() {
                if (snumChars == -1)
                    throw new InputMismatchException();
                if (curChar >= snumChars) {
                    curChar = 0;
                    try {
                        snumChars = stream.read(buf);
                    } catch (IOException e) {
                        throw new InputMismatchException();
                    }
                    if (snumChars <= 0)
                        return -1;
                }
                return buf[curChar++];
            }

            public int nextInt() {
                int c = snext();
                while (isSpaceChar(c)) {
                    c = snext();
                }
                int sgn = 1;
                if (c == '-') {
                    sgn = -1;
                    c = snext();
                }
                int res = 0;
                do {
                    if (c < '0' || c > '9')
                        throw new InputMismatchException();
                    res *= 10;
                    res += c - '0';
                    c = snext();
                } while (!isSpaceChar(c));
                return res * sgn;
            }

            public long nextLong() {
                int c = snext();
                while (isSpaceChar(c)) {
                    c = snext();
                }
                int sgn = 1;
                if (c == '-') {
                    sgn = -1;
                    c = snext();
                }
                long res = 0;
                do {
                    if (c < '0' || c > '9')
                        throw new InputMismatchException();
                    res *= 10;
                    res += c - '0';
                    c = snext();
                } while (!isSpaceChar(c));
                return res * sgn;
            }

            public int[] nextIntArray(int n) {
                int a[] = new int[n];
                for (int i = 0; i < n; i++) {
                    a[i] = nextInt();
                }
                return a;
            }

            public String readString() {
                int c = snext();
                while (isSpaceChar(c)) {
                    c = snext();
                }
                StringBuilder res = new StringBuilder();
                do {
                    res.appendCodePoint(c);
                    c = snext();
                } while (!isSpaceChar(c));
                return res.toString();
            }

            public String nextLine() {
                int c = snext();
                while (isSpaceChar(c))
                    c = snext();
                StringBuilder res = new StringBuilder();
                do {
                    res.appendCodePoint(c);
                    c = snext();
                } while (!isEndOfLine(c));
                return res.toString();
            }

            public double nextDouble() {
                return (Double.parseDouble(readString()));
            }

            public boolean isSpaceChar(int c) {
                if (filter != null)
                    return filter.isSpaceChar(c);
                return c == ' ' || c == '\n' || c == '\r' || c == '\t' || c == -1;
            }

            private boolean isEndOfLine(int c) {
                return c == '\n' || c == '\r' || c == -1;
            }

            public interface SpaceCharFilter {
                public boolean isSpaceChar(int ch);
            }
        }
    static private PrintWriter out = new PrintWriter(new BufferedOutputStream(System.out));
    static int mod = 1000000000 + 7;

}
