import java.util.Scanner;

public class MyMain {

	public static void main(String[] args) {

		Scanner sc = new Scanner(System.in);
		int n = Integer.parseInt(sc.nextLine());

		if (n == 2) {
			System.out.println(1);
			sc.close();
			return;
		}

		if (n == 3) {
			System.out.println(0);
			sc.close();
			return;
		}

		System.out.println((n % 2 == 0) ? (n - 3) / 2 : (n - 3) / 2);

		sc.close();
	}

}