import java.util.Scanner;

public class Main{
  public static void main(String[] args){
    Scanner sc=new Scanner(System.in);
    String s=sc.next();
    System.out.println(fun(s));
  }
  public static int fun(String s){
    int len=s.length(),res=0;
    for(int i=0;i<len/2;i++){
      if(s.charAt(i)!=s.charAt(len-1-i)) res++;
    }
    return res;
  }
}

/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