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

int main() {
  long N, M;
  cin >> N >> M;
  if (M % 2 && M % 5) return cout << -1 << "\n", 0;
  int n = to_string(N).size();
  int m = to_string(M).size();
  while (n < m) N *= 10, n++;
  while (m < n) M *= 10, m++;
  if (N / M == (double) N / M) return cout << N / M << "\n", 0;
  string ans = lexical_cast<string> ((double) N / M + 1e-15);
  cout << ans.at((int) ans.find("000") - 1) << "\n";
}