I think there are at least 3 ways to address this task:
1. Take a guess.
Given there is n==1,2,3,4...
the result is 1,2,5,8...
To see if you can conclude any rule with this regard.
2. Refer maths.
I guess there could be some sort of calculus or permutation and combination approaches to get it down.
3. Use algorithm and data structure.
It can be simply converted into a binary tree on which there are 2 branches with the length is 1 or 2 steps attached to each tree nodes.
Then within the tree height is n steps extend the every branches of this tree.
Finally only need to count how many tree leafs in total which is the result of this task.