n, m, c = map(int, input().split())
A = list(map(int, input().split()))
B = list(map(int, input().split()))
A.sort()
B.sort()
ans = 0
for a in A:
    left = 0
    right = len(B) - 1
    mid = 0
    while right >= left:
        mid = left + (right - left) // 2
        if B[mid] * a > c:
            right = mid - 1
        else:
            left = mid + 1
    mid = left + (right - left) // 2
    ans += m - (mid + 1)

print(ans / (n * m))