Calculate Magnetic Flux

HansBug is presented with a strange magnetic field in the form of a polygon. The polygon is described by a sequence of $N$ points representing its vertices in order. The magnetic flux in this context is defined as the area of the polygon. You can compute the area using the shoelace formula:

$$\text{Area} = \frac{1}{2} \left| \sum_{i=1}^{N} \left( x_i y_{i+1} - x_{i+1} y_i \right) \right|, $$

where $x_{N+1}=x_1$ and $y_{N+1}=y_1$. Your task is to calculate and output the magnetic flux (i.e. the area) of the polygon.

inputFormat

The first line contains an integer $N$ ($3\leq N\leq 10^5$), the number of vertices of the polygon. Each of the following $N$ lines contains two integers $x$ and $y$ ($-10^9\leq x,y\leq 10^9$), which are the coordinates of the vertices given in order.

outputFormat

Output a single number, which is the magnetic flux (area) of the polygon. Print the result with one digit after the decimal point.

sample

4
0 0
4 0
4 3
0 3

12.0

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