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 in = new Scanner(System.in);
		int n = in.nextInt();
		int n1 = n / 2;
		int cnt = 0;

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

}
