import java.util.Scanner;

public class Main {

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

		StringBuffer sb = new StringBuffer();
		for(int i = S.length(); i > 0; i--) {
			sb.append(S.charAt(i - 1));
		}
		String R = sb.toString();

		int count = 0;
		for(int i = 0; i < S.length(); i++) {
			if(S.charAt(i) != R.charAt(i)) {
				count++;
			}else {
				continue;
			}
		}
		System.out.println(count / 2);
	}

}
/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