How many rolls of wallpaper did you buy

A guy asks his neighbor in an apartment building: “Mr Trepper, you live directly above me and you have the same 2-room apartment as I do. How many rolls of wallpaper did you buy when you moved in?

“We got 18 rolls,” answers the neighbor.

Two months later the guy meets his neighbor again and says, “It’s really funny – I put the wallpaper on everywhere and I still had 10 rolls left over.”

Neighbor smiles, “Yeah, so did we.“