import java.io.*;
import java.util.*;
import java.util.Map.Entry;

import static java.lang.Math.*;

public class Ans implements Runnable {

	public static void main(String args[]) throws Exception {
		Ans s = new Ans();
		s.run();

	}

	void debug(Object... o) {
		System.out.println(Arrays.deepToString(o));
	}	
	
	InputReader sc = new InputReader(System.in);
	PrintWriter pw = new PrintWriter(System.out);
	
	public void run() {
		
		int T = sc.nextInt();
		while(T-- > 0){
			int n = sc.nextInt();
			String line = sc.readString();
			char[] s = new char[n+1];
			for(int i = 0; i < n;i ++){
				s[i+1] = line.charAt(i);
			}
			
			pw.println(solve(s, 1, n, 'a'));			
		}
		
		pw.close();
	}
	
	static int solve(char[] s, int si, int ei, char c){
		
		if(ei == si){			
			return (s[si] == c) ? 0 : 1;
		}
		
		int first = 0, last = 0;
		int mid = ((ei-si+1)/2) + si;
		for(int i = si; i <= ei; i++){
			first += (i < mid && s[i] != c) ? 1 : 0;
			last += (i >= mid && s[i] != c) ? 1 : 0;
		}
		return min(first+solve(s, mid, ei, (char)(c + 1)), last + solve(s, si, mid-1, (char)(c+1)));
	}
	
	

	
	static class InputReader {
		private InputStream stream;
		private byte[] buf = new byte[1024];
		private int curChar;
		private int numChars;
		private 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 String nextLine() {
			BufferedReader br = new BufferedReader(new InputStreamReader(System.in));
			String stock = "";
			try {
				stock = br.readLine();
			} catch (IOException e) {
				e.printStackTrace();
			}
			return stock;
		}

		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);
		}
	}
}