def read_data():
    N = int(input())
    L = list(map(int, input().split()))
    K = int(input())
    return N, L, K

N, L, K = read_data()

eps = 10**-10

left = max(L) / K
right = min (sum(L) / K, max(L)) + eps

while right - left > eps:
	piv = 0
	mid = left + (right - left) / 2
	for i in range(N):
		piv += L[i] / mid
	if piv >= K:
		left = mid + 1
	else:
		right = mid - 1
print(mid)