There is a junior high school with three grades, each grade has 300 students, a total of 900 students. The school has formulated a plan to double the enrollment of the existing 900 students, that is, it has decided to recruit 100 more students each year than the previous year starting from the enrollment of new students next year. It is known that the number of students in each class remains unchanged from freshman enrollment to graduation, so how many years later can the enrollment expansion plan be completed.
有一所初中有三个年级，每个年级为300人，共900名学生．该校制定了一个比现有900名学生增加一倍的招生计划，即决定从明年新生入学开始，每年招新生比前一年多100人．已知每一届学生从新生入学到毕业人数不变，那么多少年后才能完成这个扩大招生的计划．
<result>4</result>