Trapped in the Haybales

Farmer John has received a shipment of N large hay bales (1≤N≤100,000), and placed them at various locations along the road leading to his barn. Unfortunately, he completely forgets that Bessie the cow is out grazing along the road, and she may now be trapped within the bales!

Each bale j has a size Sj and a position Pj giving its location along the one-dimensional road. Bessie the cow can move around freely along the road, even up to the position at which a bale is located, but she cannot cross through this position. As an exception, if she runs in the same direction for D units of distance, she builds up enough speed to break through and permanently eliminate any hay bale of size strictly less than D. Of course, after doing this, she might open up more space to allow her to make a run at other hay bales, eliminating them as well.

 

Bessie can escape to freedom if she can eventually break through either the leftmost or rightmost hay bale. Please compute the total area of the road consisting of real-valued starting positions from which Bessie cannot escape.

 

输入的第一行包含一个整数N。以下N行每行描述一捆干草包，包含两个

整数，分别表示它的大小和位置，每个数的范围是1...10^9。所有的位置是不同

的。