import java.util.*;
import java.io.*;
import java.math.*;
 
public class Main {
    static PrintWriter out;
    static Reader in;
    public static void main(String[] args) throws IOException {
        //out = new PrintWriter(new File("out.txt"));
        //PrintWriter out = new PrintWriter(System.out);
        //in = new Reader(new FileInputStream("in.txt"));
        //Reader in = new Reader();
        input_output();
        Main solver = new Main();
        solver.solve();
        out.flush();
        out.close();
 
    }
 
    static int INF = (int)2e9+5;
    static int maxn = (int)1e5+5;
    static int mod=(int)1e9+7 ;
    static int n, m, t, q, k;

    static int[][] pre;
    static String s;

    void solve() throws IOException{
        t = in.nextInt();

        while (t --> 0) {
            n = in.nextInt();
            s = in.next();

            pre = new int[n][26];
            for (int i = 0; i < n; i++) {
                pre[i][s.charAt(i)-'a']++;
            }
            for (int i = 1; i < n; i++) {
                for (int j = 0; j < 26; j++) {
                    pre[i][j] += pre[i-1][j];
                }
            }

            int ans = rec(0, n-1, 0);
            out.println(ans);
        }

    }
 
    //<>

    static int rec(int l, int r, int c) {
        if (r == l) {
            if (s.charAt(l)-'a' == c) return 0;
            else return 1;
        }

        int mid = (l+r)/2;
        int second = r-mid-pre[r][c]+(mid+1 == 0 ? 0 : pre[mid][c]);
        int first = mid-l+1-pre[mid][c]+(l == 0 ? 0: pre[l-1][c]);
        return Math.min(first+rec(mid+1, r, c+1), second + rec(l, mid, c+1));
    }

    static class Node implements Comparable<Node>{
        int v, id;

        Node (int v, int id) {
            this.v = v;
            this.id = id;
        }

        public int compareTo(Node o) {
            if (this.v != o.v) return this.v - o.v;
            return o.id - this.id;
        }
    }
    
    static class Reader {
 
        private InputStream mIs;
        private byte[] buf = new byte[1024];
        private int curChar;
        private int numChars;
    
        public Reader() {
            this(System.in);
        }
    
        public Reader(InputStream is) {
            mIs = is;
        }
    
        public int read() {
            if (numChars == -1) {
                throw new InputMismatchException();
    
        }
            if (curChar >= numChars) {
                curChar = 0;
                try {
                    numChars = mIs.read(buf);
                } catch (IOException e) {
                    throw new InputMismatchException();
                }
                if (numChars <= 0) {
                    return -1;
                }
            }
            return buf[curChar++];
        }
    
        public String nextLine() {
            int c = read();
            while (isSpaceChar(c)) {
                c = read();
            }
            StringBuilder res = new StringBuilder();
            do {
                res.appendCodePoint(c);
                c = read();
            } while (!isEndOfLine(c));
            return res.toString();
        }
    
        public String next() {
            int c = read();
            while (isSpaceChar(c)) {
                c = read();
            }
            StringBuilder res = new StringBuilder();
            do {
                res.appendCodePoint(c);
                c = read();
            } while (!isSpaceChar(c));
            return res.toString();
        }
    
        double nextDouble()
        {
            return Double.parseDouble(next());
        }
    
        public long nextLong() {
            int c = read();
            while (isSpaceChar(c)) {
                c = read();
            }
            int sgn = 1;
            if (c == '-') {
                sgn = -1;
                c = read();
            }
            long 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 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 boolean isSpaceChar(int c) {
            return c == ' ' || c == '\n' || c == '\r' || c == '\t' || c == -1;
        }
    
        public boolean isEndOfLine(int c) {
            return c == '\n' || c == '\r' || c == -1;
        }
 
    }
    static void input_output() throws IOException {
        File f = new File("in.txt");
        if(f.exists() && !f.isDirectory()) { 
            in = new Reader(new FileInputStream("in.txt"));
        } else in = new Reader();
        f = new File("out.txt");
        if(f.exists() && !f.isDirectory()) {
            out = new PrintWriter(new File("out.txt"));
        } else out = new PrintWriter(System.out);
    }
}