from bisect import bisect_right


def main():
    n, m, c = map(int, input().split())
    a = [int(k) for k in input().split()]
    b = [int(k) for k in input().split()]
    ans = 0
    b.sort()
    for aa in a:
        ans += bisect_right(b, c // aa)
    print(1 - ans / n / m)


if __name__ == '__main__':
    main()