n,m,c = map(int, input().split())
a = list(map(int, input().split()))
b = list(map(int, input().split()))
b.sort()
from bisect import bisect_right,bisect_left

res = 0
for aa in a:
    if aa > c:
        res += 1/n
        continue
    tmp = c/aa
    pos = bisect_right(b,tmp)
    res += (1/n)*((m-pos)/m)

print(res)