import bisect
N,M,C = map(int,input().split())
lsa = list(map(int,input().split()))
lsb = list(map(int,input().split()))
lsa.sort()
lsb.sort()
k = 0
for i in range(M):
    f = C//lsb[i]
    l = bisect.bisect_right(lsa,f)
    k += N-l
print(k/(N*M))