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 N=sc.nextInt();
	System.out.println((N+1)/2-1);
}

}
