[Usaco2016 Dec]Lasers and Mirrors

时间限制:10s      空间限制:128MB

题目描述

For some reason, Farmer John's cows always seem to be running laser light shows.For their latest sho
w, the cows have procured a large powerful laser -- so large, in fact, that they cannot seem to move
 it easily from the location where it was delivered. They would like to somehow send the light from 
the laser to the barn on the other side of FJ's property. Both the laser and the barn can be conside
red to be located at points in the 2D plane on a map of FJ's farm. The cows plan to point the laser 
so that it sends a beam of light out either horizontally or vertically (i.e., aligned with the x or 
y axes). They will then bounce this beam off a number of mirrors to direct it to the barn.On the far
m there are N fence posts (1≤N≤100,000) located at distinct 2D points (also distinct from the lase
r and the barn) at which the cows can mount mirrors. The cows can choose not to mount a mirror on a 
fence post, in which case the laser would simply pass straight over the top of the post without chan
ging direction. If the cows do mount a mirror on a fence post, they align it diagonally like / or \ 
so that it will re-direct a horizontal beam of light in a vertical direction or vice versa.Please co
mpute the minimum possible number of mirrors the cows need to use in order to re-direct the laser to
 the barn.
出于某种原因,农夫约翰的奶牛总是喜欢使用激光。奶牛们搞到了一个强大的激光发射器,它很重,所以不能移动
它的位置。奶牛想用它照射到约翰的谷仓的另一端。众所周知,光沿直线传播,所以必须通过镜子的反射来使光线
发生偏折。激光射出的方向只能是平行于x轴或y轴的。农场上有N(1<=N<=100000)个坑位于平面直角坐标系上的
不同点(和激光发射器的位置和目标点的位置也不相同)。奶牛可以把镜子安装在坑里(当然也可以不安)。镜子
可以摆成“/”或者“\”。由物理知识可以知道,一道平行于x轴的光射到镜子上一定会平行于y轴射出,平行于y
轴的光一定会平行于x轴射出。现在给你激光发射器的坐标、目标点的坐标和所有坑的坐标,请你用最少的镜子来
实现奶牛的梦想。


输入格式

The first line of input contains 5 space-separated integers: N,xL,yL,xB,yB 
where (xL,yL) is the location of the laser and (xB,yB) is the location of the barn. 
All coordinates are between 0 and 1,000,000,000
The next N lines each contain the xx and y locations of a fence post,
both integers in the range 0…1,000,000,000
第一行包含5个正整数,分别是N,x1,y1,x2,y2,N的意义如题所述,(x1,y1)为激光发射器的坐标,(x2,y2)为目标点的坐标。
0<=x1,y1,x2,y2<=1,000,000,000
接下来N行,每行两个整数x,y表示坑的坐标。
0<=x,y<=1,000,000,000


输出格式

output the minimum number of mirrors needed to direct the laser to the barn,
or -1 if this is impossible to do.
最少安装多少个镜子。如果无论怎样都无法完成,输出-1。


样例输入

4 0 0 7 2
3 2
0 2
1 6
3 0

样例输出

1

提示

没有写明提示


题目来源

Gold