import java.io.OutputStream;
import java.io.IOException;
import java.io.InputStream;
import java.io.PrintWriter;
import java.util.InputMismatchException;
import java.io.IOException;
import java.io.InputStream;

/**
 * Built using CHelper plug-in
 * Actual solution is at the top
 *
 * @author Dhanin Gupta
 */
public class Main {
    public static void main(String[] args) {
        InputStream inputStream = System.in;
        OutputStream outputStream = System.out;
        InputReader in = new InputReader(inputStream);
        PrintWriter out = new PrintWriter(outputStream);
        DAGoodString solver = new DAGoodString();
        solver.solve(1, in, out);
        out.close();
    }

    static class DAGoodString {
        int[][] pre;

        public int recur(char[] a, int st, int en, char ch) {

            if (st == en) {
                if (pre[en][ch - 97] - pre[st - 1][ch - 97] == 1)
                    return 0;
                else
                    return 1;
            }

            int m = (st + en) / 2;

            int ch1 = ((m - st + 1) - (pre[m][ch - 97] - pre[st - 1][ch - 97]));
            int ch2 = (en - (m + 1) + 1 - (pre[en][ch - 97] - pre[m][ch - 97]));

            return Math.min(ch1 + recur(a, m + 1, en, (char) (ch + 1)), ch2 + recur(a, st, m, (char) (ch + 1)));
        }

        public void solve(int testNumber, InputReader in, PrintWriter w) {
            int t = in.nextInt();
            while (t-- > 0) {
                int n = in.nextInt();
                char[] a = (" " + in.next()).toCharArray();
                pre = new int[n + 1][26];
                for (int i = 1; i <= n; i++) {
                    for (int j = 0; j < 26; j++) {
                        pre[i][j] = pre[i - 1][j];
                    }
                    pre[i][a[i] - 97] += 1;
                }

                int ans = recur(a, 1, n, 'a');
                w.println(ans);
            }
        }

    }

    static class InputReader {
        private InputStream stream;
        private byte[] buf = new byte[1024];
        private int curChar;
        private int numChars;
        private InputReader.SpaceCharFilter filter;

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

        public int read() {
            if (numChars == -1)
                throw new InputMismatchException();

            if (curChar >= numChars) {
                curChar = 0;
                try {
                    numChars = stream.read(buf);
                } catch (IOException e) {
                    throw new InputMismatchException();
                }

                if (numChars <= 0)
                    return -1;
            }
            return buf[curChar++];
        }

        public int nextInt() {
            int c = read();

            while (isSpaceChar(c))
                c = read();

            int sgn = 1;

            if (c == '-') {
                sgn = -1;
                c = read();
            }

            int res = 0;
            do {
                if (c < '0' || c > '9')
                    throw new InputMismatchException();
                res *= 10;
                res += c - '0';
                c = read();
            }
            while (!isSpaceChar(c));

            return res * sgn;
        }

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

            return res.toString();
        }

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

        public String next() {
            return readString();
        }

        public interface SpaceCharFilter {
            public boolean isSpaceChar(int ch);

        }

    }
}

