import java.util.*;
class Main {
	public static void main(String[] args) {
		Scanner sc = new Scanner(System.in);
		
		String str = sc.next();
		char[] s = str.toCharArray(); 
		int cnt = 0;

		for(int i=0;i<str.length() / 2;i++){
			if(s[i] != s[str.length() - 1 - i] ){
				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