Shopping spree. Greg is shopping in a large urban depart-

ment store with many floors. He enters the store on the

middle floor from a skyway, and he goes directly to the

credit department. After making sure his credit is good, he

goes up three floors to the housewares department. Then

he goes down five floors to the sporting goods department.

Next, he goes up six floors to the media department. Finally,

he

goes

down ten floors to the main entrance to the store on

the first floor at ground level and leaves to go to another store.

How many floors does this department store have if it has no

basement?