import java.util.Scanner;
 
public class Main {
	public static void main(String[] args) {
      Scanner sc = new Scanner(System.in);
      String S =sc.next();
      int s =S.length();
      int b =0;
       
      for(int i=0; i<s/2; i++){
        if(S.charAt(i)!=S.charAt(s-i-1)){
        b++;
      }
      }
      System.out.print(b);
    }
}
/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