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 = Integer.parseInt(sc.next());

		//		int[] d = new int[n];
		//
		//		for (int i = 0; i < n; i++) {
		//			d[i] = Integer.parseInt(sc.next());
		//		}
		//		int ans = 0;
		//
		//		for (int i = 0; i < n; i++) {
		//			for (int j = i+1; j < n; j++) {
		//				ans += d[i] * d[j];
		//			}
		//		}

		System.out.println((n - 1) / 2);
		sc.close();
	}
}