#include <bits/stdc++.h>
using namespace std;
long long cnt(long long limit, vector<long long>& a, vector<long long>& b) {
  long long ans = 0;
  int j = (int)b.size() - 1;
  for (auto i : a) {
    while (j >= 0 and 1.0l * b[j] * i > limit) j--;
    ans += j + 1;
  }
  return ans;
}
vector<long long> get(vector<int>& x) {
  vector<long long> ans = {1};
  for (auto i : x) {
    vector<long long> temp;
    long long cur = 1;
    while (true) {
      for (auto j : ans) {
        if (1.0l * cur * j <= 1e18) temp.emplace_back(cur * j);
      }
      if (1.0l * cur * i <= 1e18)
        cur *= i;
      else
        break;
    }
    ans = temp;
  }
  return ans;
}
void solve() {
  int n;
  cin >> n;
  vector<int> p(n);
  for (int i = 0; i < n; i++) cin >> p[i];
  sort((p).begin(), (p).end());
  vector<int> a, b;
  for (int i = 0; i < n; i++) {
    if (i % 2)
      a.push_back(p[i]);
    else
      b.push_back(p[i]);
  }
  vector<long long> c = get(a);
  vector<long long> d = get(b);
  sort((c).begin(), (c).end());
  sort((d).begin(), (d).end());
  long long k;
  cin >> k;
  long long mina = 0;
  long long maxa = 1e18 + 1;
  while (maxa - mina > 1) {
    long long mid = mina + (maxa - mina) / 2;
    if (cnt(mid, c, d) < k)
      mina = mid;
    else
      maxa = mid;
  }
  cout << maxa << '\n';
}
int main() {
  ios_base::sync_with_stdio(false);
  cin.tie(0);
  cout.tie(0);
  int t = 1;
  while (t--) {
    solve();
  }
  return 0;
}
