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){
		try(Scanner sc = new Scanner(System.in)) {

			int n = sc.nextInt();

			if ( n % 2 == 0 ) {
				System.out.println(n/2 - 1);
			}
			else {
				System.out.println(n/2);
			}
			
		}
	}
}