import java.util.*;

public class Main{

	public static void main(String[] args){
		Scanner sc = new Scanner(System.in);
		String S = sc.next();
		int Len = S.length();
		int count = 0;

			for(int i=0; i<Len/2; i++){
				char ch1 = S.charAt(i);
				char ch2 = S.charAt(Len -1 -i);
			
				if(ch1 != ch2){
					count++;
				}
			}
			System.out.println(count);

	}
}
/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