n, m, c = map(int, input().split())
a = sorted(map(int, input().split()))
b = sorted(map(int, input().split()))
cnt = 0
i = 0
for y in reversed(b):
    while i < n and a[i] * y <= c:
        i += 1
    cnt += i
tot = n * m
cnt = tot - cnt
print(cnt / tot)