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 x = scanner.nextInt();
		scanner.close();

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