package No300台;

import java.util.Scanner;

public class Main{
	public static void main(String[] args) {
		solver();
	}

	static void solver() {
		Scanner sc = new Scanner(System.in);
		int a = sc.nextInt();
		if (a < 7) {
			System.out.println(-1);
			return;
		} else {
			for (int i = 8; i <= 100; i++) {
				if (a == a / i * i + a % i) {
					if (a / i == 1 && a % i == 7) {
						System.out.println(i);
						return;
					}
				}
			}
			System.out.println(-1);
			return;
		}
	}
}