#include <iostream>
#include <vector>
#include <cstdio>
#include <sstream>
#include <map>
#include <string>
#include <algorithm>
#include <queue>
#include <cmath>

using namespace std;

long long cut(vector<long long> &l, double L){
	long long ret = 0;
	L = 1.0/L;
	for(int i=0; i<l.size(); ++i){
		long long tmp = l[i]*L + 1e-9;
		ret += tmp;
	}
	return ret;
}

double bs(vector<long long> &l, long long K){
	
	double lb = 0;
	double ub = *max_element(l.begin(), l.end());
	
	const double EPS = 1e-10;

	int count = 0;
	while( fabs(ub - lb) > EPS){
		count++;
		
		if(count > 1100){
			break;
		}
		double med = (lb+ub)*0.5;
		long long tmp = cut(l, med);
		if( tmp < K ){
			ub = med;
		}else{
			lb = med;
		}
	}
	return ub;
}

int main(){
	int n;
	cin >> n;
	vector<long long> l(n);
	for(int i=0; i<n; i++) cin >> l[i];
	long long k;
	cin >> k;
	printf("%.10f\n", bs(l,k));
	return 0;
}