Let's do the 3D version now 😛

- A 1-story castle is just a 1x1x1 block.
- An n-story castle is made of four (n-1)-story castles, placed in a square with adjacent castles one unit apart, with a layer of blocks above them connecting them.

Given n > 0, return the number of 1x1x1 blocks in an n-story castle.

Image in comment.