import java.util.*;

public class Main {
    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);
        String S = sc.nextLine();
        char[] Sc = S.toCharArray();

        int index = Sc.length/2;
        int count = 0;

        for(int i=0; i < index ; i++) {
            if(Sc[i] != Sc[(Sc.length-1)-i]){
               count++;
           }
        }
        System.out.println(count);

    }
}

/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