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 scanner = new Scanner(System.in);
        int input = scanner.nextInt();
        System.out.println((input - 1) / 2);
    }
}
