import bisect
from fractions import Fraction

N, M, C = map(int, input().split())
A = list(map(int, input().split()))
B = list(map(int, input().split()))
B.sort()

p = 0
for a in A:
    c = Fraction(C, a)
    p += M-bisect.bisect_right(B, c) / (N*M)
print(p)