from bisect import bisect_right


def main():
    N, M, C = map(int, input().split())
    a = list(map(int, input().split()))
    b = list(map(int, input().split()))

    a.sort()
    b.sort()

    patterns = 0

    for a_elm in a:
        idx = bisect_right(b, C/a_elm)
        patterns += len(b) - idx

    print(patterns / (N*M))


if __name__ == "__main__":
    main()