import java.util.Scanner;
class Main {
  public static void main(String[] args) {
    Scanner sc = new Scanner(System.in);
		
    String S = sc.next();
    String li[] = S.split("");
    
    int i = 0;
    int count = 0;
    int N = li.length;
    while(i<N-i){
      if(!li[i].equals(li[N-1-i])){
        count++;
      }
      i++;
    }
    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