n, m, c = map(int, input().split())
A = list(map(int, input().split()))
B = list(map(int, input().split())) + [c + 1]
A.sort()
B.sort()

lose = 0
p = 0
for a in A[::-1]:
    while a * B[p] <= c:
        p += 1
    lose += p
print((n * m - lose) / (n * m))