#include <bits/stdc++.h>
#pragma GCC optimize(2)
using namespace std;
const int inf = 0x3f3f3f3f;
const long long INF = 0x3f3f3f3f3f3f3f3fLL;
const double PI = acos(-1.0);
const long double eps = 1e-8;
const int mod = 1e9 + 7;
const int maxn = 2e5 + 100;
const int N = 2e5 + 100;
const int M = (1 << 20) + 10;
template <class T>
inline void read(T &x) {
  x = 0;
  char c = getchar();
  bool f = 0;
  for (; !isdigit(c); c = getchar()) f ^= c == '-';
  for (; isdigit(c); c = getchar()) x = x * 10 + (c ^ 48);
  x = f ? -x : x;
}
template <class T>
inline void write(T x) {
  if (x < 0) {
    putchar('-');
    x = -x;
  }
  T y = 1;
  int len = 1;
  for (; y <= x / 10; y *= 10) ++len;
  for (; len; --len, x %= y, y /= 10) putchar(x / y + 48);
}
long long qpow(long long a, long long b, long long mod) {
  long long ans = 1;
  while (b) {
    if (b & 1) ans = (ans * a) % mod;
    b >>= 1;
    a = (a * a) % mod;
  }
  return ans;
}
const long long Mx = 1e18;
long long n, k;
vector<long long> a[2], v[2];
void dfs(int id, long long cur, int pos, long long mx) {
  if (pos == a[id].size()) {
    v[id].push_back(cur);
    return;
  }
  long long val = 1, tmp = a[id][pos];
  while (1) {
    dfs(id, cur * val, pos + 1, mx);
    if (1.0 * cur * val * tmp > 1.0 * mx) break;
    val *= tmp;
  }
}
long long check(long long mid) {
  long long res = 0;
  int j = 0;
  for (int i = v[0].size() - 1; i >= 0; --i) {
    while (j < v[1].size() && v[1][j] <= mid / v[0][i]) ++j;
    res += j;
  }
  return res;
}
int main() {
  ios::sync_with_stdio(false);
  cin.tie(0);
  cout.tie(0);
  cin >> n;
  for (int i = 1; i <= n; ++i) {
    long long x;
    cin >> x;
    a[i & 1].push_back(x);
  }
  cin >> k;
  dfs(0, 1, 0, Mx);
  dfs(1, 1, 0, Mx);
  sort(v[0].begin(), v[0].end());
  sort(v[1].begin(), v[1].end());
  long long ans = -1;
  long long l = 1, r = 1e18;
  while (l <= r) {
    long long mid = (l + r) >> 1;
    if (check(mid) >= k)
      ans = mid, r = mid - 1;
    else
      l = mid + 1;
  }
  cout << ans << "\n";
  return 0;
}
