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[] arg) {
		Scanner sc = new Scanner(System.in);

		int n = sc.nextInt();
		int a, b;
		int m = 0;

		for (int i = 1; i <= n / 2; i++) {
			a = i;
			b = n - a;
			if (a == b) continue;
			//System.out.println(a + " " + b);
			m++;
		}

		System.out.println(m);
	}
}
