import sys input=sys.stdin.readline def I(): return int(input()) def MI(): return map(int, input().split()) def LI(): return list(map(int, input().split())) N,M,C=MI() A=LI() B=LI() A.sort() B.sort() import bisect ans=0 for i in range(N): a=A[i] t=C/a num=bisect.bisect_right(B,t) ans+=M-num print('{:.10f}'.format(ans/(N*M)))