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;

public class Main {

	public static void main (String[] args)  {

		Scanner sc = new Scanner(System.in);
		int a = sc.nextInt();
		System.out.println((a-1)/2);

	}

}
