#include <bits/stdc++.h>
using namespace std;
long long a[30], n;
vector<long long> xs, ys;
long long calc(long long x) {
  int r = ys.size() - 1;
  long long ret = 0, s;
  for (int i = 0; i < xs.size(); i++) {
    s = xs[i];
    if (s > x) break;
    while (ys[r] > x / s) r--;
    ret = ret + r + 1;
  }
  return ret;
}
int main() {
  scanf("%lld", &n);
  for (int i = 1; i <= n; i++) scanf("%d", &a[i]);
  sort(a + 1, a + 1 + n);
  for (int i = 1; i <= n / 2; i++) swap(a[i], a[n - i + 1]);
  long long mx = 1e18, s;
  xs.clear();
  ys.clear();
  xs.push_back(1);
  ys.push_back(1);
  for (int t = 1; t <= n / 2; t++) {
    s = a[t];
    for (int i = 0; i < xs.size(); i++)
      if (s <= mx / xs[i]) xs.push_back(xs[i] * s);
  }
  for (int t = n / 2 + 1; t <= n; t++) {
    s = a[t];
    for (int i = 0; i < ys.size(); i++)
      if (s <= mx / ys[i]) ys.push_back(ys[i] * s);
  }
  sort(xs.begin(), xs.end());
  sort(ys.begin(), ys.end());
  xs.erase(unique(xs.begin(), xs.end()), xs.end());
  ys.erase(unique(ys.begin(), ys.end()), ys.end());
  long long l = 1, r = 1e18;
  long long k;
  scanf("%lld", &k);
  while (l <= r) {
    long long mid = (l + r) / 2;
    if (calc(mid) < k)
      l = mid + 1;
    else
      r = mid - 1;
  }
  printf("%lld\n", l);
  return 0;
}
