import numpy as np
eps = 1e-9
N = int(input())
L = np.array(input().split(), dtype=np.float64)
K = int(input())


l = 0.
r = 1e9 + eps
for _ in range(100):
    m = (r + l) * 0.5
    if (L / m + eps).astype(np.int32).sum() >= K:
        l = m
    else:
        r = m

print("{:.10f}".format((l + r) * 0.5))