#include <bits/stdc++.h>
long long int nd[1000000] = {0};
int compare(const void *a, const void *b) {
  if (*(long long int *)a - *(long long int *)b < 0) return -1;
  if (*(long long int *)a - *(long long int *)b > 0) return 1;
  return 0;
}
int main() {
  long long int num, k;
  scanf("%lld %lld", &num, &k);
  int x = sqrt(num);
  int c = 0, i = 0, flag = 1;
  for (i = 1; i <= x; i++) {
    if (num % i == 0) {
      nd[c++] = i;
      if ((long long int)i * i == num) break;
      nd[c++] = num / i;
    }
  }
  qsort(nd, c, sizeof(long long int), compare);
  if (k > c)
    printf("-1");
  else
    printf("%lld", nd[k - 1]);
  return 0;
}
