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.Scanner;

class Main {
  public static void main(String[] args) {
    Scanner stdIn = new Scanner(System.in);

    int n = stdIn.nextInt();

    if (n % 2 == 0) {
      System.out.println((n - 1) / 2);
    } else {
      System.out.println(n / 2);
    }
  }
}
