N, M, C = map(int, input().split())
a = list(map(int, input().split()))
b = list(map(int, input().split()))

b.sort()
ans = 0
for ai in a:
	left = -1
	right = M
	while(right - left > 1):
		mid = int(left + (right - left) // 2)
		if(ai * b[mid] > C): right = mid
		else: left = mid
	ans += M - right
print(ans / (N*M))