#include <bits/stdc++.h>
using namespace std;

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

	int ans = -1;
	for( int X = 2; X <= A; X++ ) {
		vector<int> n;
		int a = A;
		while( a > 0 ) {
			n.push_back( a % X );
			a /= X;
		}
		if( n.size() == 2 && n[0] == 7 && n[1] == 1 ) {
			ans = X;
			break;
		}
	}

	cout << ans << endl;
}