import java.util.*;
 
public class Main {
    public static void main(String[] args){
        Scanner sc = new Scanner(System.in);
        String s = sc.next();
        int N = s.length();
        int ans = 0;
        
        for(int i = 0; i < N/2; i++){
            if(s.charAt(i) != s.charAt(N-i-1)){
                ans++;
            }
        }
        System.out.println(ans);
    }
}