import java.util.*;

public class Main {
	public static void main(String[] args) {
		Scanner scan = new Scanner(System.in);
		
		int ans = 0;
		String str = scan.next();
		for(int i=0;i<str.length()/2;i++) {
			if(str.charAt(i) != str.charAt(str.length()-1-i)) {
				ans += 1;
			}
		}
		System.out.println(ans);
	}

}



/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