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);
		
		// Nの入力
		int N = sc.nextInt();
		
		// 結果の出力
		int result = (int) (N % 2 == 0 ? Math.floor(N/2) - 1 : Math.floor(N/2));
		System.out.println(result);
		
		sc.close();
	}

}
