system './' . $exe_name;

BEGIN {
	$source_name = 'my_tmp.cpp';
	$exe_name = $^O eq 'MSWin32' ? 'my_a.exe' : 'my_a.out';
	return if -e $exe_name;
	open my $fh, '>', $source_name;
	print $fh <<'CODE';
#line 9

#include <string>
#include <vector>
#include <algorithm>
#include <numeric>
#include <set>
#include <map>
#include <queue>
#include <iostream>
#include <sstream>
#include <cstdio>
#include <cmath>
#include <ctime>
#include <cstring>
#include <cctype>
#include <cassert>
#include <limits>
#include <functional>

#include <immintrin.h>

#define rep(i,n) for(int (i)=0;(i)<(int)(n);++(i))
#define rer(i,l,u) for(int (i)=(int)(l);(i)<=(int)(u);++(i))
#define reu(i,l,u) for(int (i)=(int)(l);(i)<(int)(u);++(i))
#if defined(_MSC_VER) || __cplusplus > 199711L
#define aut(r,v) auto r = (v)
#else
#define aut(r,v) __typeof(v) r = (v)
#endif
#define each(it,o) for(aut(it, (o).begin()); it != (o).end(); ++ it)
#define all(o) (o).begin(), (o).end()
#define pb(x) push_back(x)
#define mp(x,y) make_pair((x),(y))
#define mset(m,v) memset(m,v,sizeof(m))
#define INF 0x3f3f3f3f
#define INFL 0x3f3f3f3f3f3f3f3fLL
using namespace std;
typedef vector<int> vi; typedef pair<int,int> pii; typedef vector<pair<int,int> > vpii; typedef long long ll;
template<typename T, typename U> inline void amin(T &x, U y) { if(y < x) x = y; }
template<typename T, typename U> inline void amax(T &x, U y) { if(x < y) x = y; }

inline long long sum_trunc_mul(const int *A, int N, double s_inv) {
#ifdef _MSC_VER
	__m256d inv = _mm256_set1_pd(s_inv);
	int i = 0;
	__m256d sum = _mm256_setzero_pd();
	for(; i + 3 < N; i += 4) {
		__m128i a = _mm_loadu_si128(reinterpret_cast<const __m128i*>(A + i));
		__m256d a_d = _mm256_cvtepi32_pd(a);
		__m256d prod = _mm256_mul_pd(a_d, inv);
		__m256d truncated = _mm256_round_pd(prod, (_MM_FROUND_TO_ZERO |_MM_FROUND_NO_EXC));
		sum = _mm256_add_pd(sum, truncated);
	}
	long long res = 0;
	rep(k, 4) res += (ll)sum.m256d_f64[k];
	for(; i < N; ++ i)
		res += (ll)(A[i] * s_inv);
	return res;
#endif
}

int main() {
	int N;
	while(~scanf("%d", &N)) {
		vector<int> A(N);
		rep(i, N) scanf("%d", &A[i]);
		long long K;
		scanf("%lld", &K);
		const double EPS = 1e-9;
		double l = 0, u = 1e9;
		while(l + EPS < u && l * (1 + EPS) < u) {
			double mid = (l + u) / 2;
			double inv = 1. / mid;
			long long sum = 0;
			rep(i, N) sum += (ll)(A[i] * inv);
			if(sum >= K) l = mid; else u = mid;
		}
		printf("%.10f\n", (l + u) / 2);
	}
	return 0;
}

CODE
	system "g++ -m64 -O2 -lm -march=native -std=c++11 $source_name -o $exe_name 2> my_compile.log";
	if($? != 0) {
		open(my $fh, '<', 'my_compile.log');
		while(<$fh>) { print STDERR $_; }
		die 'compile error';
	}
}