3988：Selecting courses

A new Semester is coming and students are troubling for selecting courses. Students select their course on the web course system. There are n courses, the i^th course is available during the time interval (A_i,B_i). That means, if you want to select the i^th course, you must select it after time A_i and before time B_i. A_i and B_i are all in minutes. A student can only try to select a course every 5 minutes, but he can start trying at any time, and try as many times as he wants. For example, if you start trying to select courses at 5 minutes 21 seconds, then you can make other tries at 10 minutes 21 seconds, 15 minutes 21 seconds,20 minutes 21 seconds… and so on. A student can’t select more than one course at the same time. It may happen that no course is available when a student is making a try to select a course
You are to find the maximum number of courses that a student can select.

There are no more than 100 test cases.
The first line of each test case contains an integer N. N is the number of courses (0 < N <= 300)
Then N lines follows. Each line contains two integers A_i and B_i (0 <= A _i < B_i <=1000), meaning that the i^th course is available during the time interval (A_i,B_i).
The input ends by N = 0.

For each test case output a line containing an integer indicating the maximum number of courses that a student can select.