#include <iostream>
#include <cstdio>

using namespace std;

int main() {
	int a;
	cin >> a;
	int ans = -1;
	for (int x = 8; x <= 100; x++) {
		if (a % x == 7 && a / x == 1) {
			ans = x;
			break;
		}
	}
	cout << ans << endl;
	return 0;
}