import java.util.Scanner;

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

	String S = sc.next();
	char[] c = S.toCharArray();

	int count = 0;
	for (int i = 0, j = c.length - 1; i < c.length / 2; i++, j--) {
	    if (c[i] != c[j])
		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