from bisect import bisect_left


def main():
    n, m, c = map(int, input().split())
    arr = list(map(int, input().split()))
    b = list(map(int, input().split()))
    b.sort()

    ans = 1 - sum([bisect_left(b, c // a + 1) for a in arr]) / (n * m)
    print(ans)


if __name__ == "__main__":
    main()