import java.util.*;

class Main {
    public static void main(String[] args){
        Scanner sc = new Scanner(System.in);
        String S = sc.nextLine();
        int len = S.length();
        int count = 0;

        for(int i=0; i<len/2; i++){
            if(S.charAt(i) != S.charAt(len-i-1)){
                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