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;

/**
 * <p>Description: </p>
 *
 * @author yong.zhang
 * @version 1.0.0
 * @date 2019/11/14 9:17
 */
public class Main {
    public static void main(String[] args) {
        Scanner in = new Scanner(System.in);
        int N = in.nextInt();
        System.out.println((N - 1) / 2);
    }
}
