package com.company;

import java.io.BufferedReader;
import java.io.IOException;
import java.io.InputStreamReader;

public class Main {

    public static void main(String[] args) throws IOException {
        BufferedReader br = new BufferedReader(new InputStreamReader(System.in));
        br.readLine();
        String s = br.readLine();
        int add = 0;
        int i = 0;
        int j = s.length() -1;
        while (i < j) {
            if(s.charAt(i) != s.charAt(j)) {
                add++;
            }
            else {
                j--;
            }
            i++;
        }
        System.out.println(add);
    }
}