import java.util.*;

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

		String s = sc.nextLine();

		int x = 0;

		for(int i=0;i < s.length()/2 ;i++){
			if(!(s.charAt(i) == s.charAt(s.length()-i-1)))
				x+=1;
		}
		System.out.println(x);

	}
}
/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