import java.util.*;
public class Main {
	public static void main(String[] args){
		Scanner sc = new Scanner(System.in);
		// 整数の入力
		String a = sc.next();
        String b[] = a.split("");
      
        int count = 0;
        int half = a.length() / 2;
        for(int i = 0; i < half;i++){
          if(!b[i].equals(b[a.length() - 1 - i])){
            count += 1;
          }
        }
        
      	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