Trahen is building a fence that is 6 feet high and 18 feet wide to put around his garden. He has two sizes of boards that he can use. One is 2 feet by 6 feet and the other is 4 feet by 6 feet. Trahen can put the boards either horizontally or vertically and he has lots of boards of each size. How many different ways can he build the fence?

Details and assumptions
Trahen only builds 1 fence that is 6 feet high by 18 feet wide.

Hint: recurrence --> fibonacci. This question is reposted from brilliant.