#include <bits/stdc++.h>
int compare(const void *a, const void *b) {
  long long la = *(long long *)a;
  long long lb = *(long long *)b;
  return la == lb ? 0 : la < lb ? -1 : 1;
}
int main() {
  static long long aa[26880];
  long long n, a;
  int k, m;
  scanf("%lld%d", &n, &k);
  m = 0;
  for (a = 1; a <= n / a; a++)
    if (n % a == 0) {
      aa[m++] = a;
      if (a != n / a) aa[m++] = n / a;
    }
  if (k > m) {
    printf("-1\n");
    return 0;
  }
  qsort(aa, m, sizeof *aa, compare);
  printf("%lld\n", aa[k - 1]);
  return 0;
}
