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 sc = new Scanner (System.in);
		int a = sc.nextInt();
		if(a % 2 == 0) {
			int ans = a / 2;
			ans = ans - 1;
			System.out.println(ans);
		}else {
			int ans = a / 2;
			System.out.println(ans);
		}
	}
}