#include <iostream>
#include <stdio.h>
#include <string.h>
#include <vector>
#include <string>
#include <set>
#include <map>
#include <algorithm>
#include <numeric>
#include <queue>
#include <cassert>
#include <cmath>
#include <bitset>
using namespace std;

int main() {
  int n;
  cin >> n;
  vector<double> l(n);
  for (int i = 0; i < n; i++) cin >> l[i];
  long long k;
  cin >> k;

  double ok = 0;
  double ng = 1e10;
  for (int j = 0; j < 100; j++) {
    double x = (ok+ng)/2.0;
    long long cnt = 0;
    for (int i = 0; i < n; i++) cnt += l[i]/x;
    //cout << "x = " << x << ", cnt = " << cnt << endl;
    if (cnt >= k) ok = x;
    else ng = x;
  }
  printf("%.15llf\n", ok);
}