from bisect import bisect_left


def ceildiv(a: int, b: int) -> int:
    return (a + b - 1) // b


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

B.sort()
cnt = 0
for a in A:
    x = ceildiv(C+1, a)
    p = bisect_left(B, x)
    cnt += M - p

ans = cnt / (N * M)
print(ans)