import java.util.Scanner;

public class Main{
public static void main(String[] args){
Scanner scan=new Scanner(System.in);
String str = scan.next();
  
 String[] num=str.split("");
  
  int i=0;
  int sum=0;
  for(i=0;i<(num.length)/2;i++){
  if(num[i].equals(num[num.length-1-i])){
  sum=sum+1;
  }
  }
  System.out.println((num.length/2)-sum);
  
}
}
/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