import java.util.*;
 
public class Main {
	public static void main (String[] args) {
		Scanner in = new Scanner(System.in);
		
		String s = in.next();
		int nam = s.length();
		char[] s_chars = s.toCharArray();
		
		int nam2 = nam/2;
		int cnt = 0;
		for(int i = 0;i<nam2;i++){
		    if(s_chars[i] != s_chars[nam-i-1]){
		        cnt++;
		    }
		}
       
       System.out.print(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