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) {
    Scanner scan = new Scanner(System.in);
    int N = scan.nextInt();
    int ans = N%2==0 ? N/2-1 : N/2;
    System.out.println(ans);
  }
}