import java.util.*;
import java.io.*;

public class Main {
  public static void main(String args[]) {
    Scanner sc = new Scanner(System.in);
    String s = sc.next();
    int c = 0;
    int l = s.length();
    for(int i=0;i<l/2;i++) {
      c+= s.charAt(i)==s.charAt(l-i-1)?0:1;
    }
    System.out.println(c);
  }
}
/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