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# Area

For a given polygon g, computes the area of the polygon.

g is represented by a sequence of points $p_1$, $p_2$,…, $p_n$ where line segments connecting pi and pi+1 (1 ≤ in−1) are sides of g. The line segment connecting pn and p1 is also a side of the polygon.

Note that the polygon is not necessarily convex.

## Input

The input consists of coordinates of the points p1,…, pn in the following format:

The first integer n is the number of points. The coordinate of a point pi is given by two integers xi and yi. The coordinates of points are given in the order of counter-clockwise visit of them.

## Output

Print the area of the polygon in a line. The area should be printed with one digit to the right of the decimal point.

## Constraints

• 3 ≤ n ≤ 100
• -10000 ≤ $x_i,y_i$ ≤ 10000
• No point will occur more than once.
• Two sides can intersect only at a common endpoint.