#include<iostream>
#include<string>
#include<algorithm>
#include<functional>
#include<vector>
#include<set>
#include<queue>
#include<list>
#include<stdio.h>
#include<stdlib.h>
#include<map>

#define rep(i,a,b) for(int i=a;i<b;i++)
#define rrep(i,a,b) for(int i=a;i>=b;i--)
#define fore(i,a) for(auto &i:a)
#define all(x) (x).begin(),(x).end()

using namespace std;

int main() {
	int A;
	cin >> A;

	int X = A - 7;

	int r = A%X;

	int q = -1;
	if (X != 0) {
		q = A / X;
	}

	if (q == 1 && r == 7) {
		cout << X << endl;
	}
	else {
		cout << "-1" << endl;
	}

	return 0;
}