The computer science department runs a mentoring center to undergraduate students with their programming assignments. the lab has a coordinator and several teachers to assist the students. the waiting area of the center has several chairs. initially, all the chairs are empty, the coordinator is waiting for students to arrive and all the teachers are waiting for students to be assigned by coordinator. a student who arrives at the center seeking sits in an empty chair in the waiting area and waits to be called for tutoring. if no chairs are available, the student will come back at a later time. if the coordinator sees a student waiting, then the coordinator wakes up an idle teacher, if there is one, and assigns the student to the teacher. if all the teachers are busy assisting students, the coordinator waits for a teacher to get free and then assigns a waiting student to the teacher. a teacher after assisting a student, waits for the next student to be assigned to him.