import java.util.ArrayList;
import java.util.Iterator;
import java.util.List;
import java.util.Scanner;
import java.util.TreeMap;

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

        String S = sc.next();

        char[] str = S.toCharArray();

        int count = 0;
        for (int i = 0; i < str.length / 2; i++) {
            if (str[i] == str[str.length - 1 - i]) {
                continue;
            } else {
                count++;
            }
        }

        System.out.println(count);

        sc.close();
    }
}
/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