/*
・メモ
	g++ -O2 -S -m64 -march=sandybridge
・何にはまっていたのか？
	こっちの環境でコンパイルした時、配列Aのアライメントが仮定されたコードが出力されたようだ。
	しかし、向こうの環境ではアライメントされていなかった。よってアライメントエラーでREとなっていた。
*/

#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>

#ifdef MY_LOCAL_RUN
#include <immintrin.h>
#endif

#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; }

const double *g_A;
int g_N;
double g_s_inv;
long long g_res;

#ifdef MY_LOCAL_RUN
extern "C" void sum_trunc_mul();
__declspec(noinline) void sum_trunc_mul() {
	const double *A = g_A;
	int N = g_N;
	double s_inv = g_s_inv;

	__m256d inv = _mm256_set1_pd(s_inv);
	int i = 0;
	__m256d sum = _mm256_setzero_pd();
	for(; i + 3 < N; i += 4) {
		__m256d a_d = _mm256_load_pd(A + i);
		__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);
	}
	double sum4[4];
	_mm256_storeu_pd(sum4, sum);
	long long res = 0;
	rep(k, 4) res += (ll)sum4[k];
	for(; i < N; ++ i)
		res += (ll)(A[i] * s_inv);

	g_res = res;
}
#else
extern "C" void sum_trunc_mul();
__asm(
".text\n"
"sum_trunc_mul:\n"
".L_FB4544:\n"
"	subq	$40, %rsp\n"
"	movl	g_N(%rip), %r9d\n"
"	vmovsd	g_s_inv(%rip), %xmm2\n"
"	movq	g_A(%rip), %r8\n"
"	vmovddup	%xmm2, %xmm3\n"
"	vinsertf128	$1, %xmm3, %ymm3, %ymm3\n"
"	cmpl	$3, %r9d\n"
"	jle	.L_10\n"
"	leal	-4(%r9), %ecx\n"
"	xorl	%eax, %eax\n"
"	vxorpd	%xmm1, %xmm1, %xmm1\n"
"	shrl	$2, %ecx\n"
"	movl	%ecx, %edx\n"
"	addq	$1, %rdx\n"
"	salq	$5, %rdx\n"
"	.p2align 4,,10\n"
".L_6:\n"
"	vmulpd	(%r8,%rax), %ymm3, %ymm0\n"
"	addq	$32, %rax\n"
"	vroundpd	$11, %ymm0, %ymm0\n"
"	vaddpd	%ymm0, %ymm1, %ymm1\n"
"	cmpq	%rdx, %rax\n"
"	jne	.L_6\n"
"	leal	4(,%rcx,4), %r11d\n"
".L_5:\n"
"	vmovupd	%ymm1, (%rsp)\n"
"	xorl	%eax, %eax\n"
"	xorl	%ecx, %ecx\n"
"	movq	%rsp, %r10\n"
".L_7:\n"
"	vcvttsd2siq	(%r10,%rax), %rdx\n"
"	addq	$8, %rax\n"
"	addq	%rdx, %rcx\n"
"	cmpq	$32, %rax\n"
"	jne	.L_7\n"
"	cmpl	%r11d, %r9d\n"
"	jle	.L_8\n"
"	subl	$1, %r9d\n"
"	movslq	%r11d, %rdx\n"
"	leaq	(%r8,%rdx,8), %rax\n"
"	subl	%r11d, %r9d\n"
"	leaq	1(%rdx,%r9), %rdx\n"
"	leaq	(%r8,%rdx,8), %r8\n"
"	.p2align 4,,10\n"
".L_9:\n"
"	vmulsd	(%rax), %xmm2, %xmm0\n"
"	addq	$8, %rax\n"
"	vcvttsd2siq	%xmm0, %rdx\n"
"	addq	%rdx, %rcx\n"
"	cmpq	%r8, %rax\n"
"	jne	.L_9\n"
".L_8:\n"
"	movq	%rcx, g_res(%rip)\n"
"	vzeroupper\n"
"	addq	$40, %rsp\n"
"	ret\n"
".L_10:\n"
"	vxorpd	%xmm1, %xmm1, %xmm1\n"
"	xorl	%r11d, %r11d\n"
"	jmp	.L_5\n"
"\n");

#endif

int main() {
	int N;
	{	scanf("%d", &N);
		char *A_d_buf = new char[N * 8 + 32];
		double *A_d = reinterpret_cast<double*>((reinterpret_cast<uintptr_t>(A_d_buf) | 31) + 1);
		rep(i, N) {
			int A;
			scanf("%d", &A);
//			A = 5;
			A_d[i] = A;
		}
		long long K;
		scanf("%lld", &K);
//		K = 8;
		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;
			g_A = &A_d[0], g_N = N, g_s_inv = inv;
			sum_trunc_mul();
			long long sum = g_res;
			if(sum >= K) l = mid; else u = mid;
		}
		printf("%.10f\n", (l + u) / 2);
		delete[] A_d_buf;
	}
	return 0;
}