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

    a = 0
    b = max(L)

    d = 5 * 10 ** -10

    while b - a > d:
        c = (a + b) / 2
        n = sum(int(l / c) for l in L)
        if n >= K:
            a = c
        else:
            b = c

    print((b - a) / 2)


def main():
    solve()


if __name__ == '__main__':
    main()