from bisect import bisect_right

n, m, c = map(int, input().split())
a = list(map(int, input().split()))
a.sort()
b = list(map(int, input().split()))
b.sort()
ans = 0
for i in range(n):
    ai = a[i]

    c = m - bisect_right(b, c / ai)
    ans += c
ans /= n * m
print(ans)