import java.io.*;
    import java.util.*;
    import java.math.*;
    import java.lang.*;

    import static java.lang.Math.*;

    public class Main implements Runnable
    {
        static class InputReader
        {
            private InputStream stream;
            private byte[] buf = new byte[1024];
            private int curChar;
            private int numChars;
            private SpaceCharFilter filter;
            private BufferedReader br=new BufferedReader(new InputStreamReader(System.in));

            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 String nextLine()
            {
                String str = "";
                try
                {
                    str = br.readLine();
                }
                catch (IOException e)
                {
                    e.printStackTrace();
                }
                return str;
            }
            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 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 double nextDouble()
            {
                int c = read();
                while (isSpaceChar(c))
                    c = read();
                int sgn = 1;
                if (c == '-')
                {
                    sgn = -1;
                    c = read();
                }
                double res = 0;
                while (!isSpaceChar(c) && c != '.')
                {
                    if (c == 'e' || c == 'E')
                        return res * Math.pow(10, nextInt());
                    if (c < '0' || c > '9')
                        throw new InputMismatchException();
                    res *= 10;
                    res += c - '0';
                    c = read();
                }
                if (c == '.')
                {
                    c = read();
                    double m = 1;
                    while (!isSpaceChar(c))
                    {
                        if (c == 'e' || c == 'E')
                            return res * Math.pow(10, nextInt());
                        if (c < '0' || c > '9')
                            throw new InputMismatchException();
                        m /= 10;
                        res += (c - '0') * m;
                        c = read();
                    }
                }
                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);
            }
        }

        public static void main(String args[]) throws Exception
        {
            new Thread(null, new Main(),"Main",1<<26).start();
        }
        //User-defined functions goes here...
        public int calc(String s, char c){
            if(s.length() == 1)
                if(s.charAt(0) == c)
                    return 0;
                else
                    return 1;

            int mid = s.length()/2;
            int cntl = calc(s.substring(0, mid), (char)((int)c+1));
            int count = 0;
            for(int i=mid; i<s.length(); i++)
                if(s.charAt(i) == c)
                    count++;
            cntl += s.length()/2 - count;

            int cntr = calc(s.substring(mid, s.length()), (char)((int)c+1));
            count = 0;
            for(int i=0; i<mid; i++)
                if(s.charAt(i) == c)
                    count++;

            cntr += s.length()/2 - count;

            return Math.min(cntl, cntr);

        }

        public void run()
        {
            InputReader sc= new InputReader(System.in);
            PrintWriter w= new PrintWriter(System.out);

            //Your code goes here...

            int test = sc.nextInt();
            for (int q0=0; q0<test; q0++) {
                int n = sc.nextInt();
                String s = sc.next();

                System.out.println(calc(s, 'a'));


            }

            w.close();
        }
    }