#include <stdio.h>
#include <algorithm>
#include <iostream>
#include <string>
#include <vector>
#include <functional>
#include <map>
#include <iomanip>
#include <math.h> 
#include <stack>
#include <queue>
#include <bitset>
#include <cstdlib>
#include <tuple>
#include <cctype>
#include <ctype.h>
#include <set>
#include <sstream>

using namespace std;

int main(){
	int i, j;
	int a;

	cin >> a;

	int ans = 99999;

	for (i = 2; i <= 100; i++) {

		int num = a;
		vector<string>list;
		string ss;

		while (true)
		{
			list.push_back(to_string(num % i));
			num /= i;
			if (num == 0) {
				break;
			}

		}

		for (j = list.size() - 1; j >= 0; j--) {
			ss += list[j];
		}

		if (ss == "17") {
			ans = i;
		}

	}


	if (ans == 99999) {
		cout << -1 << endl;
	}
	else {
		cout << ans << endl;
	}








	getchar();
	getchar();
	return 0;
}