The new version of Lianliankan is a game you love to play with. The rules of the game are as follows:
Give you n boxes, each with its own properties, two integers x, y. (1≤x,y≤10000)
You can only start from the box with property 1 then connect the next boxes which has property 2 and so on.The properties of boxes you have chosen are increased.
Each box can only be selected once.When you choose the box,you can only use the one property of it.
In other words, you can only select a box with property 1 at first, then connect to a box with property 2, and then connect to a box with property 3.......
How many boxes can you connect ?(The more the better.ass we can)
The first line is one integer n(1≤n≤1000000)
Then n lines follow,each line contains two integers,which are the box‘s properties
Using fast I/O to avoid Time-Limit-Exceeded
Output the answer we want