import java.util.Scanner;

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

		}
		System.out.println(change);
	}
}
/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