import bisect
N,M,C = map(int,input().split()); Error = pow(10,-9)
A = list(map(int,input().split()))
B = list(map(int,input().split()))
B.sort()
ALL = N*M
cnt = 0
for i in range(N):
  need = C/A[i] + Error
  ok = M - bisect.bisect_right(B,need)
  cnt += ok
  #print(ok)
ans = cnt/ALL
print(ans)