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