from bisect import bisect_left

n, m, c = map(int, input().split())
A = list(map(int, input().split()))
B = list(map(int, input().split()))

A.sort()
B.sort()
ans = 0
for i in range(n):
    res = c // A[i] + 1
    idx = bisect_left(B, res)
    ans += m - idx
print(ans / n / m)