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

  }
}
/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