## USACO 2012 US Open, Gold Division

## Problem 1. Tied Down

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**Not submitted**

Problem 1: Tied Down [Brian Dean, 2012]
As we all know, Bessie the cow likes nothing more than causing mischief on
the farm. To keep her from causing too much trouble, Farmer John decides
to tie Bessie down to a fence with a long rope. When viewed from above,
the fence consists of N posts (1 <= N <= 10) that are arranged along
vertical line, with Bessie's position (bx, by) located to the right of this
vertical line. The rope FJ uses to tie down Bessie is described by a
sequence of M line segments (3 <= M <= 10,000), where the first segment
starts at Bessie's position and the last ends at Bessie's position. No
fence post lies on any of these line segments. However, line segments may
cross, and multiple line segments may overlap at their endpoints.
Here is an example of the scene, viewed from above:
To help Bessie escape, the rest of the cows have stolen a saw from the
barn. Please determine the minimum number of fence posts they must cut
through and remove in order for Bessie to be able to pull free (meaning she
can run away to the right without the rope catching on any of the fence posts).
All (x,y) coordinates in the input (fence posts, Bessie, and line segment
endpoints) lie in the range 0..10,000. All fence posts have the same x
coordinate, and bx is larger than this value.
PROBLEM NAME: tied
INPUT FORMAT:
* Line 1: Four space-separated integers: N, M, bx, by.
* Lines 2..1+N: Line i+1 contains the space-separated x and y
coordinates of fence post i.
* Lines 2+N..2+N+M: Each of these M+1 lines contains, in sequence, the
space-separated x and y coordinates of a point along the rope.
The first and last points are always the same as Bessie's
location (bx, by).
SAMPLE INPUT (file tied.in):
2 10 6 1
2 3
2 1
6 1
2 4
1 1
2 0
3 1
1 3
5 4
3 0
0 1
3 2
6 1
INPUT DETAILS:
There are two posts at (2,3) and (2,1). Bessie is at (6,1). The rope goes
from (6,1) to (2,4) to (1,1), and so on, ending finally at (6,1). The shape
of the rope is the same as in the figure above.
OUTPUT FORMAT:
* Line 1: The minimum number of posts that need to be removed in order
for Bessie to escape by running to the right.
SAMPLE OUTPUT (file tied.out):
1
OUTPUT DETAILS:
Removing either post 1 or post 2 will allow Bessie to escape.

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