from bisect import *
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)
j = 0
cnt = 0
for a in A:
    while j < M and a * B[j] > C:
        j += 1
    cnt += j
print(cnt / N / M)