import java.util.Scanner;

class Main {
  public static void main(String[] args) {
    Scanner stdIn = new Scanner(System.in);
    String s = stdIn.next();
    String[] str = s.split("");
    int n = s.length();
    int count = 0;

    for (int i = 0; i < n / 2; i++) {
      if (!str[i].equals(str[n - i - 1])) {
        count++;
      }
    }

    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