The Elevator

Avi works in a multi-story building, on the top floor. His friends Benny and Gil also work in the same building. Every day, in the morning and evening, Avi goes to greet them.
In the morning Avi enters the building,

• goes up 21 floors in the elevator to Benny's office,
• walks down to Gil's office,
• goes up 17 floors in the elevator to his office.

After work,

• Avi goes down 9 floors in the elevator to Benny's office,
• walks down to Gil's office,
• goes down 13 floors in the elevator and exits the building.

How many floors are in the building? (Reminder - the ground floor is also a floor)