n, m, c = map(int, input().split())
a = sorted(list(map(int, input().split())))
b = sorted(list(map(int, input().split())), reverse=True)
ans = 0
index = 0
for x in a:
    while index < m and x * b[index] > c:
        index += 1
    ans += index
print(ans / (n * m))