import java.util.Scanner;

public class Main {
	static char[] AZ = {'A','Z'};
	public static void main(String[] args) {
		Scanner sc = new Scanner(System.in);
		String s = sc.next();
		sc.close();
		char[] c = s.toCharArray();
		int cnt = 0;
		int len = c.length/2;
		int max = c.length;
		for(int i=0;i<len;i++) {
//			System.out.println(c[i] + ";" + c[len+(c.length%2)+i]);
//			System.out.println(c[i] + ";" + c[max-1-i]);
			if(c[i]!=c[max-1-i]) {
				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