import bisect

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

C = 0
for a in A:
    t = ((c + 1) + a - 1) // a
    C += m - bisect.bisect_left(B, t)

print(C / (n * m))