#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; }

#ifdef MY_LOCAL_RUN
__declspec(noinline) long long sum_trunc_mul(const double *A, int N, double 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);
    return res;
}
#else
long long sum_trunc_mul(const double *A, int N, double s_inv);
__asm(
".intel_syntax noprefix\n"
"_Z13sum_trunc_mulPKdid:\n"
".LMY_FB3893:\n"
"	push	rbp\n"
"	mov	rbp, rsp\n"
"	sub	rsp, 80\n"
"	mov	QWORD PTR 16[rbp], rcx\n"
"	mov	DWORD PTR 24[rbp], edx\n"
"	vmovsd	QWORD PTR 32[rbp], xmm2\n"
"	sub	rsp, 352\n"
"	mov	rax, rsp\n"
"	add	rax, 31\n"
"	shr	rax, 5\n"
"	sal	rax, 5\n"
"	mov	rdx, QWORD PTR 32[rbp]\n"
"	mov	QWORD PTR -32[rbp], rdx\n"
"	vbroadcastsd	ymm0, QWORD PTR -32[rbp]\n"
"	vmovapd	YMMWORD PTR 256[rax], ymm0\n"
"	mov	DWORD PTR -4[rbp], 0\n"
"	vxorpd	xmm0, xmm0, xmm0\n"
"	vmovapd	YMMWORD PTR 288[rax], ymm0\n"
"	jmp	.LMY_8\n"
".LMY_12:\n"
"	mov	edx, DWORD PTR -4[rbp]\n"
"	movsx	rdx, edx\n"
"	lea	rcx, 0[0+rdx*8]\n"
"	mov	rdx, QWORD PTR 16[rbp]\n"
"	add	rdx, rcx\n"
"	mov	QWORD PTR -40[rbp], rdx\n"
"	mov	rdx, QWORD PTR -40[rbp]\n"
"	vmovapd	ymm0, YMMWORD PTR [rdx]\n"
"	vmovapd	YMMWORD PTR 224[rax], ymm0\n"
"	vmovapd	ymm0, YMMWORD PTR 224[rax]\n"
"	vmovapd	YMMWORD PTR 128[rax], ymm0\n"
"	vmovapd	ymm0, YMMWORD PTR 256[rax]\n"
"	vmovapd	YMMWORD PTR 96[rax], ymm0\n"
"	vmovapd	ymm0, YMMWORD PTR 128[rax]\n"
"	vmulpd	ymm0, ymm0, YMMWORD PTR 96[rax]\n"
"	vmovapd	YMMWORD PTR 192[rax], ymm0\n"
"	vroundpd	ymm0, YMMWORD PTR 192[rax], 11\n"
"	vmovapd	YMMWORD PTR 160[rax], ymm0\n"
"	vmovapd	ymm0, YMMWORD PTR 288[rax]\n"
"	vmovapd	YMMWORD PTR 64[rax], ymm0\n"
"	vmovapd	ymm0, YMMWORD PTR 160[rax]\n"
"	vmovapd	YMMWORD PTR 32[rax], ymm0\n"
"	vmovapd	ymm0, YMMWORD PTR 64[rax]\n"
"	vaddpd	ymm0, ymm0, YMMWORD PTR 32[rax]\n"
"	vmovapd	YMMWORD PTR 288[rax], ymm0\n"
"	add	DWORD PTR -4[rbp], 4\n"
".LMY_8:\n"
"	mov	edx, DWORD PTR -4[rbp]\n"
"	add	edx, 3\n"
"	cmp	edx, DWORD PTR 24[rbp]\n"
"	jl	.LMY_12\n"
"	lea	rdx, -80[rbp]\n"
"	mov	QWORD PTR -48[rbp], rdx\n"
"	vmovapd	ymm0, YMMWORD PTR 288[rax]\n"
"	vmovapd	YMMWORD PTR [rax], ymm0\n"
"	mov	rdx, QWORD PTR -48[rbp]\n"
"	vmovapd	ymm0, YMMWORD PTR [rax]\n"
"	vmovupd	YMMWORD PTR [rdx], ymm0\n"
"	mov	QWORD PTR -16[rbp], 0\n"
"	mov	DWORD PTR -20[rbp], 0\n"
"	jmp	.LMY_13\n"
".LMY_14:\n"
"	mov	eax, DWORD PTR -20[rbp]\n"
"	cdqe\n"
"	mov	rax, QWORD PTR -80[rbp+rax*8]\n"
"	vmovq	xmm1, rax\n"
"	vcvttsd2si	rax, xmm1\n"
"	add	QWORD PTR -16[rbp], rax\n"
"	add	DWORD PTR -20[rbp], 1\n"
".LMY_13:\n"
"	cmp	DWORD PTR -20[rbp], 3\n"
"	jle	.LMY_14\n"
"	jmp	.LMY_15\n"
".LMY_16:\n"
"	mov	eax, DWORD PTR -4[rbp]\n"
"	cdqe\n"
"	lea	rdx, 0[0+rax*8]\n"
"	mov	rax, QWORD PTR 16[rbp]\n"
"	add	rax, rdx\n"
"	mov	rax, QWORD PTR [rax]\n"
"	vmovq	xmm3, rax\n"
"	vmulsd	xmm0, xmm3, QWORD PTR 32[rbp]\n"
"	vcvttsd2si	rax, xmm0\n"
"	add	QWORD PTR -16[rbp], rax\n"
"	add	DWORD PTR -4[rbp], 1\n"
".LMY_15:\n"
"	mov	eax, DWORD PTR -4[rbp]\n"
"	cmp	eax, DWORD PTR 24[rbp]\n"
"	jl	.LMY_16\n"
"	mov	rax, QWORD PTR -16[rbp]\n"
"	mov	rsp, rbp\n"
"	pop	rbp\n"
"	ret\n"
".att_syntax\n"
"\n");

#endif

int main() {
    int N;
    while(~scanf("%d", &N)) {
        vector<double> A_d(N);
        rep(i, N) {
            int A;
            scanf("%d", &A);
            A_d[i] = A;
        }
        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 = sum_trunc_mul(&A_d[0], N, inv);
            if(sum >= K) l = mid; else u = mid;
        }
        printf("%.10f\n", (l + u) / 2);
    }
    return 0;
}