#include <bits/stdc++.h>
using namespace std;
const long long modu = 1e9 + 7;
int noofdig(int N) { return floor(log10(N)) + 1; }
int bits_count(unsigned int u) {
  unsigned int uCount;
  uCount = u - ((u >> 1) & 033333333333) - ((u >> 2) & 011111111111);
  return ((uCount + (uCount >> 3)) & 030707070707) % 63;
}
void solve() {
  long long int n, k;
  cin >> n >> k;
  vector<long long int> div;
  for (long long int i = 1; i * i <= n; i++) {
    if (n % i == 0) {
      div.push_back(i);
      if (i * i != n) div.push_back(n / i);
    }
  }
  sort(div.begin(), div.end());
  if ((long long int)div.size() >= k)
    cout << div[k - 1] << '\n';
  else
    cout << -1 << '\n';
}
int main() {
  ios_base::sync_with_stdio(false);
  cin.tie(NULL);
  srand(chrono::high_resolution_clock::now().time_since_epoch().count());
  int t = 1;
  while (t--) {
    solve();
  }
}
