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

/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