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 num = sc.nextInt();
		
		if(num%2 == 1)
			System.out.println(num/2);
		else
			System.out.println(num/2-1);
	}
}
