Preserving parks
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The department of environmental studies wants to monitor the parks to check if
they are being converted to housing complexes as has been alleged in several
complaints launched with them. Rather than get involved in costly litigation 
after builidings have been erected in parks, they would like to act proactively 
when they see parks disappearing.

The department wishes to design a surveillance system that is capable of 
taking aerial images and automatically detecting parks. Further, they want
their system to compute the area of each park to make sure that the park is
not being encroached upon. Your job is to write a program that does this. For
simplicity, you may assume that each park is a convex polygon (mathematically,
a polygon is convex if a line joining any two vertices is on or inside the 
polygon).

Input
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The input will contain multiple instances. Each instance has two lines. The 
first line contains a single integer, which is the number of points n 
(0 < n < 10000). The second line contains 2n floating point numbers (separated 
by a single blank space), x1,y1, x2,y2,....,xn,yn. The vertices of the 
polygon are in clockwise or counterclockwise order.

The end of the input will be indicated by a line containing 0. This case 
should not be processed.

Output
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The output should contain one line for each case. Each line should have a
single floating point number denoting the area. Each area should be rounded to 
the nearest single decimal.

Sample input
------------

4
1 0 2.1 0 2.1 -1 1 -1
0

Sample output
-------------

1.1