import java.util.Scanner;

class Main {
    public static void main(String[] args) {
      Scanner sc = new Scanner(System.in);
      String a = sc.next();
      int count = 0;
        for (int i = 0; i < a.length()/2; i++) {
          	if(a.charAt(i) != a.charAt(a.length()-i-1)){
				count += 1;
            }
        }
      System.out.print(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