import bisect

N,M,C = map(int,input().split())
a = list(map(int,input().split()))
b = list(map(int,input().split()))
b.sort()
res = 0
for i in range(N):
    limit = C//a[i]
    idx = bisect.bisect_right(b,limit)
    res += idx

print((N*M-res)/(N*M))