Frankie has a large number of wooden cubes, painted in red and green. He is building little towers with his cubes, by stacking them on top of each other.
He got bored after a while, so he introduced a rule: two red cubes may not be touching (see image).
Given n (the maximum number of cubes in a tower), determine the number m of different towers that Frankie can build.
Example: for n = 2, he can make 5 towers, so m=5:
You may assume that the input is an integer greater than zero.