import java.io.BufferedOutputStream;
import java.io.BufferedReader;
import java.io.IOException;
import java.io.InputStreamReader;
import java.io.PrintWriter;
import java.util.StringTokenizer;

public class Main {

  public static void main(String[] args) {
    Main main = new Main();
    try {
      main.run(args);
    } catch (Exception e) {
      e.printStackTrace();
    }
  }

  private void run(String[] arguments) throws Exception {
    MyScanner sc = new MyScanner();
    PrintWriter out = new PrintWriter(new BufferedOutputStream(System.out));
    String s = sc.next();
    char[] letters = s.toCharArray();

    int diff = 0;
    for (int i = 0; i < letters.length / 2; i++) {
      char font = letters[i];
      char back = letters[letters.length - i - 1];
      if (font != back) {
        diff++;
      }
    }

    out.println(diff);
    out.flush();

  }

  /*
   * Form: http://codeforces.com/blog/entry/7018
   */
  private class MyScanner {

    BufferedReader br;
    StringTokenizer st;

    MyScanner() {
      br = new BufferedReader(new InputStreamReader(System.in));
    }

    String next() {
      while (st == null || !st.hasMoreElements()) {
        try {
          st = new StringTokenizer(br.readLine());
        } catch (IOException e) {
          e.printStackTrace();
        }
      }
      return st.nextToken();
    }

    int nextInt() {
      return Integer.parseInt(next());
    }

    long nextLong() {
      return Long.parseLong(next());
    }

    double nextDouble() {
      return Double.parseDouble(next());
    }

    String nextLine() {
      String str = "";
      try {
        str = br.readLine();
      } catch (IOException e) {
        e.printStackTrace();
      }
      return str;
    }
  }
}

/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