#include "bits/stdc++.h"

using namespace std;

using ll = long long int;
using ld = long double;

using vi = vector<int>;
using vvi = vector<vi>;
using vvvi = vector<vvi>;
using vl = vector<ll>;
using vvl = vector<vl>;
using vvvl = vector<vvl>;
using vd = vector<double>;
using vvd = vector<vd>;
using vb = vector<bool>;
using vvb = vector<vb>;
using vs = vector<string>;

const int INF = (1 << 30) - 1;
const ll INF64 = ((ll)1 << 62) - 1;
const double PI = 3.1415926535897932384626433832795;

const int dx[] = { -1, 0, 1, 0 };
const int dy[] = { 0, 1, 0, -1 };

int gcd(int x, int y) { return y ? gcd(y, x % y) : x; }
ll gcd(ll x, ll y) { return y ? gcd(y, x % y) : x; }
int lcm(int x, int y) { return x / gcd(x, y) * y; }
ll lcm(ll x, ll y) { return x / gcd(x, y) * y; }

bool check(ll n, vl l, ll k, ld mid) {
	ll sum = 0;
	for (auto v : l) {
		sum += (ld)v / mid;
		if (sum >= k) {
			return true;
		}
	}
	return false;
}

double solve(ll n, vl l, ll k) {
	ld left = 0.0, right = 10000000001.0;

	int cnt = 0;
	while (left < right - 1.0) {
		ld mid = (left + right) / 2.0;
		if (check(n, l, k, mid)) {
			left = mid;
		} else {
			right = mid;
		}
	}

	return left;
}

int main() {
	/*ios::sync_with_stdio(false);
	cin.tie(0);*/

	ll n;
	cin >> n;

	vl l(n);
	for (int i = 0; i < n; i++) {
		cin >> l[i];
	}

	ll k;
	cin >> k;
	cout << fixed << setprecision(15) << solve(n, l, k) << endl;
	return 0;
}