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

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

    static class DAGoodString {
        public void solve(int testNumber, FastReader s, PrintWriter out) {
            int t = s.nextInt();
            while (t-- > 0) {
                int n = s.nextInt();
                char[] arr = s.nextString().toCharArray();
                int pow = func1(n);
                int[][] dp = new int[n][pow + 1];
                for (int i = 0; i < n; i++) {
                    Arrays.fill(dp[i], -1);
                }
                out.println(func(arr, 0, pow, pow, dp));
            }
        }

        private int func(char[] arr, int pos, int currP, int power, int[][] dp) {
            char ch = (char) ('a' + power - currP);
            if (currP == 0) {
                if (arr[pos] == ch) {
                    return 0;
                }

                return 1;
            }

            if (dp[pos][currP] != -1) {
                return dp[pos][currP];
            }

            int count = 0;
            for (int i = pos; i < pos + (1 << (currP - 1)); i++) {
                if (arr[i] != ch) {
                    count++;
                }
            }

            int ans = count + func(arr, pos + (1 << (currP - 1)), currP - 1, power, dp);
            count = 0;
            int pos1 = pos + (1 << (currP - 1));

            for (int i = pos1; i < (pos1 + (1 << (currP - 1))); i++) {
                if (arr[i] != ch) {
                    count++;
                }
            }

            return dp[pos][currP] = Math.min(ans, count + func(arr, pos, currP - 1, power, dp));
        }

        private int func1(int n) {
            int count = 0;
            while (n > 1) {
                count++;
                n /= 2;
            }

            return count;
        }

    }

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

        public FastReader(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 nextString() {
            int c = read();
            while (isSpaceChar(c)) {
                c = read();
            }
            StringBuilder res = new StringBuilder();
            do {
                if (Character.isValidCodePoint(c)) {
                    res.appendCodePoint(c);
                }
                c = read();
            } while (!isSpaceChar(c));
            return res.toString();
        }

        public boolean isSpaceChar(int c) {
            if (filter != null) {
                return filter.isSpaceChar(c);
            }
            return isWhitespace(c);
        }

        public static boolean isWhitespace(int c) {
            return c == ' ' || c == '\n' || c == '\r' || c == '\t' || c == -1;
        }

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

        }

    }
}

