import java.util.Scanner;
public class Main {
    public static void main(String[] args){
        Scanner sc = new Scanner(System.in);
        String s = sc.next();
        sc.close();
        int n = s.length();
        int k = 0;
        for(int i = 0 ; i < n / 2 ; i++ ){
            if(s.charAt(i) != s.charAt(n - i - 1)){
                k++;
            }
        }
        System.out.println(k);
    }
}
/tmp/hf-datasets-cache/medium/datasets/42704881117311-config-parquet-and-info-qiankunmu-Project_CodeNet-896c9d36/hub/datasets--qiankunmu--Project_CodeNet_Python800_and_Java250/snapshots/c899fde44a77ba1cc14973ed3de064e4ff3aadc7/Project_CodeNet_Java250.tar.gz