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

a.sort()
b.sort()

def tansaku(x):
    if x * b[-1] < C:
        return N
    end = N-1
    start = -1
    while end - start > 1:
        mid = (start + end) // 2
        if x * b[mid] >= C:
            end = mid
        else:
            start = mid
    return end
ans = 0
for i in a:
    ans += N - tansaku(i)
print(ans / (H * W))