import java.util.Scanner;

public class Main {

	public static void main(String[] args) {
		String s = "";
		Scanner sc = new Scanner(System.in);
		s = sc.next();
		int len = s.length();
		String[] ar = s.split("");
		int count = 0;
		for (int i = 0; i < len / 2; i++) {
			if (ar[i].equals(ar[len - i - 1]) == false) {
				count++;
			}
		}
		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