#include <iostream>
using namespace std;

int main(void) {
	int A, X = -1;

	cin >> A;
	for (int i = 2; A >= i * 1 + 7; i++) {
		if (A == 7 + i * 1) {
			X = i;
			break;
		}
	}

	cout << X << endl;
	return 0;
}