class Student:
    def __init__(self, student_id: int):
        self._student_id = student_id


    def student_id(self) -> int:
        return self._student_id



class Club:
    def __init__(self):
        self._students = {}


    def number_of_students(self) -> int:
        return len(self._students)


    def add_student(self, student_to_add: Student) -> None:
        self._students[student_to_add.student_id()] = student_to_add