import java.util.Scanner;

public class Main {

	public static void main(String[] args) {
      		Scanner sc = new Scanner(System.in);

		String S = sc.next();
		int length = S.length();
		char c =' ';
		char d =' ';
		int cnt = 0;

		for(int i=0;i<=length/2-1;i++) {
			c = S.charAt(i);
			d = S.charAt(length-1-i);
			if(c!=d) {
				cnt++;
			}
		}
		System.out.println(cnt);
	}

}

/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