#!/usr/bin/env perl

use strict;
use warnings;

my $idol = <>; chomp $idol;
for (my $x = 7; $x < 100; $x++) {
  my $n = $idol;
  my $ans = 0;
  while ($n > 0) {
    $ans *= 10;
    $ans += $n % $x;
    $n = int($n / $x);
  }
  if ($ans == 71) {
    print $x;
    exit;
  }
}
print -1