You have red blocks and green blocks. You want to build a tower where level 1 has 1 block, level 2 has 2 blocks, level 3 has 3 blocks, and so on.
Each level must be a single color (all red or all green). How many different towers can you build using exactly all your blocks? Before reading on, think: why can't = "ways to build first levels" work? What information is missing? Before reading the solution, try to identify the base case and recursive relationship yourself.