#include <iostream>
#include <iomanip>

const int MAX_INT = 1000000000;
const double EPS = 1e-9;

int min(int L[], int N) {
	int min_val = MAX_INT;
	for (int i=0; i<N; i++) {
		if (min_val > L[i]) min_val = L[i];
	}
	return min_val;
}

int sum(int L[], int N) {
	int sum_val = 0;
	for (int i=0; i<N; i++) sum_val += L[i];
	return sum_val;
}

bool retrievable(double x, int L[], int N, int K) {
	int max_num = 0;
	for (int i=0; i<N; i++) {
		max_num += (int)(L[i] / x);
	}
	return max_num >= K;
}

double find(double a, double b, int L[], int N, int K) {
	if (b - a < EPS) {
		return a;
	} else {
		double c = (a + b) / 2;
		if (retrievable(c, L, N, K)) {
			return find(c, b, L, N, K);
		} else {
			return find(a, c, L, N, K);
		}
	}
}

int main() {
	int N;
	std::cin >> N;
	int L[N];
	for (int i=0; i<N; i++) std::cin >> L[i];
	int K;
	std::cin >> K;
	
	double a = (double)min(L, N) / (double)K;
	double b = (double)sum(L, N) / (K-0.5);
	
	std::cout << std::fixed;
	std::cout << std::setprecision(9)
			<< find(a, b, L, N, K) << std::endl;
}