from math import floor, sqrt
def read_data():
    N = int(input())
    Ls = list(map(int, input().split()))
    K = int(input())
    return N, Ls, K

def solve(N, Ls, K):
    eps = 10**-10
    lower = max(Ls)/K - eps
    upper = max(Ls) + eps
    while upper - lower > eps:
        mid = sqrt(upper * lower)
        if sum(floor(L/mid) for L in Ls) >= K:
            lower = mid
        else:
            upper = mid
    return (upper + lower) / 2

N, Ls, K = read_data()
print(solve(N, Ls, K))