#include <bits/stdc++.h>

using namespace std;

#define FOR(i,a,b) for(int i=(a);i<(b);i++)
#define REP(i,n) FOR(i,0,n)
#define ALL(v) (v).begin(),(v).end()

template<typename A, typename B> inline bool chmax(A &a, B b) { if (a<b) { a=b; return 1; } return 0; }
template<typename A, typename B> inline bool chmin(A &a, B b) { if (a>b) { a=b; return 1; } return 0; }

typedef unsigned long long ull;
typedef long long ll;
typedef pair<int, int> pii;
typedef pair<ll, ll> pll;
typedef pair<int, pii> P;

const ll INF = 1ll<<29;
const ll MOD = 1000000007;
const double EPS  = 1e-10;

int N, L[212345];
ll K;

int main() {
	cin >> N;
	REP(i, N) scanf("%d", L + i);
	cin >> K;
	
	double r = 1e9, l = 0.0;
	REP(i, 100) {
		double m = (r + l) / 2;
		
		ll now = 0;
		REP(j, N) now += L[j] / m;
		
		if (now < K) r = m;
		else l= m;
	}
	
	printf("%.15f\n", r);
	
	return 0;
}