#include <bits/stdc++.h>
using namespace std;
template <class T> using vec = vector<T>;
// #define vec vector
typedef long long ll;
typedef vec<int> vi;
typedef vec<ll> vl;
#define M 1000000007
#define all(x) (x).begin(), (x).end()
#define rall(x) (x).rbegin(), (x).rend()
#define dbg(x) cout << #x << " = " << x << '\n'
#define db(x) #x << " = " << x << " "

// #define TEST

void realmain() {
  ll n,k;
  cin >> n;
  vl a(n);
  ll mx=0;
  for (int i=0;i<n;++i) {
    cin >> a[i];
    mx = max(mx, a[i]);
  }
  cin >> k;

  double l=0,r=mx;
  for (int p=0;p<50;++p){
    double m=(l+r)/2;
    ll sm=0;
    for (auto &i:a) sm+=i/m;
    if (sm>=k) l=m;
    else r=m;
  }
  cout << fixed << setprecision(15) << l;
}

int main() {
  ios_base::sync_with_stdio(0);
  cin.tie(0);

#define N "YUBIYUBI"
  if (fopen(N ".inp", "r")) {
    freopen(N ".inp", "r", stdin);
    freopen(N ".out", "w", stdout);
  }

  ll t = 1;
#ifdef TEST
  cin >> t;
#endif // TEST

  while (t--) {
    realmain();
  }

  return 0;
}