import java.util.Scanner;

public class Main {
    public static void main(final String[] args) {
        try (Scanner sc = new Scanner(System.in)) {
            int cnt = 0;
            String input = sc.next();
            int halfLength = input.length() / 2;
            for (int i = 0; i < halfLength; i++) {
                char c1 = input.charAt(i);
                char c2 = input.charAt(input.length() - 1 - i);
                if (c1 != c2)
                    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