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

int main()
{
    int N;
    cin >> N;
    for (int i = 8; i <= N + 1; i++)
    {
        int tmp = N;
        string s;
        while (tmp)
        {
            s += '0' + tmp % i;
            tmp /= i;
        }
        reverse(s.begin(),s.end());
        if (s == "17")
        {
            cout << i;
            return 0;
        }
    }
    cout << -1;
}