Missing Dollar

3 guys go to a hotel. They want a room together. The owner decides to rip them off and charges more for the room. He charges them $30 for one room.They each give $10 and go to their room. Afterwards, the owner feels guilty and decides to give them back the money they overpaid. He calls the bell boy, gives him $5 and tells him to take it back to the 3 guys. On his way, the bell boy remembers they never tipped him. He keeps $2 and gives each of them $1 each. In the end, they each paid $9. That's $27 for the room. The bell boy has $2. Where's the missing dollar?