When calculating the area of an irregular polygon, all the vertices are written down in order ending with the starting vertex. Then the product of first y with the second x is subtracted from the product of first x is with the second y, etc.

Question:
Write a program that calculates the area of the given polygon.

Input specification
You are given first an integer number n 0 ≤ n ≤ 10,000. Then, in the following n lines you are given coordinates of an irregular polygon. Each coordinate x and y can have any value between -20,000 and +20,000.

Output specification
Show one number with one digit double precision: Area of the given polygon.