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

A.sort()
B.sort(reverse = True)

r = 0
cnt = 0
for a in A:

    # r:a*B[r]<=Cとなるはじめのポイント
    while r < M and B[r] * a > C:
        r += 1

    cnt += r

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