Count layer by layer. 
The diagram shows some cubes of the same size stacked at a corner of a room. How many cubes are there altogether? (Note: The floor is horizontal and the two walls are vertical. There are no gaps or holes behind the visible cubes). 

- $$24$$ 
- $$25$$ 
- $$26$$ 
- $$27$$ 
- $$28$$ 