#include <iostream>
#include <vector>
using namespace std;

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

  vector<int> ans = {7,1};
  for(int x=2; x<=100; x++){
    int tmp = A;
    vector<int> hoge;
    while(tmp){
      hoge.push_back(tmp%x);
      tmp /= x;
    }

    if(hoge == ans){
      cout << x << endl;
      return 0;
    }
  }

  cout << -1 << endl;
  return 0;
}