import java.util.Scanner;
public class No395 {


	static int judge(int a) {
		int x =8;
		while(x<a) {
			if(a/x==1 && a%x==7) {
				return x;
			}
			else {
				x++;
			}

		}


		return -1;
	}

	public static void main(String[]args) {
		Scanner sc = new Scanner(System.in);
		int a = sc.nextInt();

		int ans = judge(a);

		System.out.println(ans)
;
		sc.close();
	}

}