import bisect


n, m, c = map(int, input().split())
a = list(map(int, input().split()))
b = list(map(int, input().split()))


b = sorted(b)
all_ptn = n * m
ptn = 0

for val in a:
    other = c // val + 1
    idx = bisect.bisect_left(b, other)
    ptn += m - idx
print(ptn /all_ptn)