#include <bits/stdc++.h>
using namespace std;
inline long long read() {
  long long SUM = 0, ff = 1;
  char ch = getchar();
  while (ch < '0' || ch > '9') {
    if (ch == '-') ff = -1;
    ch = getchar();
  }
  while (ch >= '0' && ch <= '9') {
    SUM = (SUM << 3) + (SUM << 1) + (ch ^ 48);
    ch = getchar();
  }
  return SUM * ff;
}
const int N = 20;
const long long inf = 1e18;
long long n, k;
long long prime[N];
vector<long long> num[2];
inline void meet_in_the_middle(int now, long long sum, int flag) {
  if (now > n) {
    num[flag].push_back(sum);
    return;
  }
  for (long long w = 1;; w *= prime[now]) {
    meet_in_the_middle(now + 2, sum * w, flag);
    if (inf / prime[now] < sum * w) return;
  }
}
int main() {
  n = read();
  for (int i = 1; i <= n; i++) prime[i] = read();
  sort(prime + 1, prime + n + 1);
  int mid = n >> 1;
  num[0].push_back(0), num[1].push_back(0);
  meet_in_the_middle(1, 1, 0);
  meet_in_the_middle(2, 1, 1);
  sort(num[0].begin(), num[0].end());
  sort(num[1].begin(), num[1].end());
  k = read();
  long long l = 1, r = inf, res = inf, l0 = num[0].size(), l1 = num[1].size();
  while (l <= r) {
    long long mid = (l + r) >> 1;
    long long cnt = 0;
    for (int i = 1, j = l1 - 1; i < l0 && j >= 1; i++, cnt += j)
      while (j && mid / num[0][i] < num[1][j]) j--;
    if (cnt < k) {
      l = mid + 1;
    } else {
      res = mid;
      r = mid - 1;
    }
  }
  printf("%lld\n", res);
  return 0;
}
