The way state do the requirements is really unique. So sometimes when you get 16, 17 or 18 only then it requires to add more late. We try and keep it simple and encourage everyone to do the 20 hour initially. Your best bet is to go to the company you took the 18 from and buy another Federal regs course. I think its a 3-hour.
Wish we could help but let us know if anything else we can assist with.