#include <bits/stdc++.h>
using namespace std;
int main() {
  long long n, k, d;
  set<long long> a;
  cin >> n >> k;
  for (d = 1; d * d <= n; d++)
    if (n % d == 0) a.insert({d, n / d});
  if (k <= a.size())
    cout << *next(a.begin(), k - 1);
  else
    cout << -1;
}
