#include "bits/stdc++.h"
using namespace std;
#define INF 1e9
#define rep(i, n) for (int i = 0; i < n; i++)
#define rep2(i, l, r) for (int i = (l); i < (r); i++)
#define ALL(x) (x).begin(), (x).end()     //昇順
#define RALL(x) (x).rbegin(), (x).rend()  // 降順
#define FOR(i, c) \
  for (__typeof((c).begin()) i = (c).begin(); i != (c).end(); ++i)
const long long mod = 1e9 + 7;
typedef long long ll;  // ll とdoubleは違う
typedef priority_queue<int> PQ;
typedef vector<long long> VL;
typedef vector<bool> VB;
typedef vector<int> VI;  // VI a(n);
typedef vector<double> VD;
typedef vector<string> VS;
typedef vector<char> VC;
typedef vector<VS> VSS;
typedef vector<VC> VCC;
typedef vector<VI> VII;  // VII a(n,vector<int>(m)) n * m
typedef vector<VL> VLL;
typedef pair<int, int> PII;
typedef map<int, int> MP;  // MP a;
typedef vector<pair<ll, ll>> PS;
template <class T, class U>  // chmax(max, a);
bool chmax(T &a, U b) {
  if (a < b) {
    a = b;
    return 1;
  }
  return 0;
}
template <class T, class U>  // chmin(min,a)
bool chmin(T &a, U b) {
  if (a > b) {
    a = b;
    return 1;
  }
  return 0;
}
template <typename T>  // operator << (cout,a);
ostream &operator<<(ostream &os, vector<T> &v) {
  os << "{";
  rep(i, (int)v.size()) { os << v[i] << (i < v.size() - 1 ? ", " : ""); }
  os << "}";
  return os;
}

// g++ -std=c++11 prac.cpp
int n;
ll a[300000];

ll num(double v){
  ll t = 0;
  rep(i,n){
    t+=a[i]/v;
  }
  return t;
}

int main() {
  ll x = 1, y = 0, z = 0, k, h, ans = 0, w, sum = 1, Max = -1e9 - 1,
     cnt = 0, Min = 1e9 + 1;
  string s, t, u;
  bool ok = true;

  cin >> n;
  rep(i, n) { cin >> a[i]; }
  cin >> k;

  double l = 0, r = 1e11, m;
  rep(i, 100) { 
    m = (l + r) / 2; 
    if(num(m) >= k) l = m;
    else r = m;
  }
  cout << setprecision(12) << m << endl;
  return 0;
}