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;

//第二回全国統一プログラミング王決定戦予選
//A	Sum of Two Integers
public class Main {

	public static void main(String[] args) {
		Scanner sc = new Scanner(System.in);
		int N = Integer.parseInt(sc.next());
		sc.close();

		int ans = (N / 2);
		if (N % 2 == 0) {
			ans--;
		}
		System.out.println(ans);
	}
}
