import java.util.Scanner;
import java.util.Arrays;

class Main{
  public static void main(String[] args){
    Scanner sc = new Scanner(System.in);
    String s = sc.next();
    char[] c = s.toCharArray();
    int n = s.length();
    
    int d = 0;
    
    for(int i=0; i<n/2; i++){
      if(c[i] != c[(n-1)-i]){
        d++;
      }
    }
    
    System.out.println(d);
    
  }
}

/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