How many ways are there to choose two distinct positive integers totaling N, disregarding the order?

Constraints

* 1 \leq N \leq 10^6
* N is an integer.

Input

Input is given from Standard Input in the following format:


N


Output

Print the answer.

Examples

Input

4


Output

1


Input

999999


Output

499999
import java.util.*;
public class Main {
  
  public static void main(String[] args) {
    int N = Integer.parseInt(new Scanner(System.in).next());
    int a = (int) Math.floor((N - 1) / 2);
    System.out.println(a);
  }
}