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2019 USACO January

Mountain View

From her pasture on the farm, Bessie the cow has a wonderful view of a mountain range on the horizon. There are n mountains in the range 1...105. If we think of Bessie's field of vision as the xy plane, then each mountain is a triangle whose base rests on the x axis. The two sides of the mountain are both at 45 degrees to the base, so the peak of the mountain forms a right angle. Mountain i is therefore precisely described by the location (xi, yi) of its peak. No two mountains have exactly the same peak location.

Bessie is trying to count all of the mountains, but since they all have roughly the same colour, she cannot see a mountain if its peak lies on or within the triangular shape of any other mountain.

Determine the number of distinct peaks, and therefore mountains, that Bessie can see.


The first line contains n (1n105). Each of the remaining n lines contains xi (0xi109) and yi (1yi109) describing the location of one mountain's peak.


Print the number of mountains that Bessie can distinguish.


In this example, Bessie can see the first and last mountain. The second mountain is obscured by the first.

Time limit 1 second
Memory limit 128 MiB
Input example #1
4 6
7 2
2 5
Output example #1
Source 2019 USACO January, Silver