During this economic crisis time, Jack has started an incredible new business related to air
travel, a parking-lot for airplane. He bought a very large land to park airplanes. However the
land is very narrow, so that the only way airplanes can go in or go out of the parking lot must
be in the Last-In First-Out fashion (see picture below). He only has spaces in the parking lot
so he cannot take some airplane at the end out so that other airplanes can move.

Because of the limitation of the parking lot, it is not possible to accommodate all requests for
parking. Each request consists of the planned arrival time and planned departure time, which
are the times the airplane arrives at the parking lot. An example below shows a request table
for 4 planes.

In this case, it is possible to accommodate airplane 1, 2, and 4. But it is not possible to
accommodate both airplanes 2 and 3.

It is possible that different planes plan to arrive or depart the parking lot at the same time. Jack
has the best crews working with him, so that they will manage to arrange the plane to the
parking lot in the best way that if it is possible to park and take out the planes they will be able
to do it. Consider another example.

Although airplane 5 and 6 arrive at the same time, Jack's crews know that airplane 5 will have
to be out before airplane 6, so when both airplanes arrive they put airplane 6 in first, following
by airplane 5.

Given a list of parking requests, you want to find the maximum number of airplanes that can be
parked in this parking lot, provided that they can only depart in the Last-In First-Out fashion.