import java.util.*;
public class Main {
	public static void main(String[] args){
		Scanner sc = new Scanner(System.in);
      
      //入力
      String S = sc.next();
      int N = S.length();
      
      char[] a =S.toCharArray();
      int count =0;
      
      //違う文字の数を探す
      for(int i =0; i<N/2; i++){
       if(a[i]!=a[N-1-i]){
         count++;
         }
      }
      //出力
       System.out.print(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