#include <iostream>
#include <algorithm>
#include <cstdio>

using namespace std;

long long int N, K;
int L[200010];

bool check(long double x)
{

    long double tmp = 0;
    for (int i = 0; i < N; i++)
    {
        tmp += L[i] / x;
    }

    return K <= tmp;
}

int main()
{
    cin >> N;
    for (int i = 0; i < N; i++)
        cin >> L[i];

    cin >> K;

    long double lb = 0;
    long double ub = 1e10;
    long double mid = 0;

    for (int i = 0; i < 100; i++)
    {
        mid = (lb + ub) / 2;
        if (check(mid))
            lb = mid;
        else
            ub = mid;
    }

    printf("%.14Lf", lb);

    return 0;
}