#include<bits/stdc++.h>
using namespace std;
#include<atcoder/all>
using namespace atcoder;
using ll = long long;
int n;
vector<ll> L;
ll k = 0;

void solve(){
	double l = 0,r = 1e18;
	for(int i = 0;i<1000;i++){
		double m = (l + r ) /2;
		ll cnt = 0;
		for(int j = 0 ;j<n;j++){
			cnt += L[j]/m;
		}
		if(cnt<k)r = m;
		else l = m;
	}
	printf("%.10lf\n",l);
}

signed main(){
	cin.tie(nullptr);
	ios::sync_with_stdio(false);
	cin >> n ;
	L = vector<ll>(n);
	for(auto &i:L)cin >> i;
	cin >> k;
	solve();
}