this is a Synchronize problem
The Sleeping-Barber Problem:
-A barbershop consists of a waiting room with n chairs and the barber room containing the barber chair.
-If there are no customers to be served, the barber goes to sleep.
-If a customer enters the barbershop and all chairs are occupied, then the customer leaves the shop. -If the barber is busy but chairs are available, then the customer sits in one of the free chairs.
-If the barber is asleep, the customer wakes up the barber.
output is the result