Area of a Polygon#60
Given the coordinates of the vertices of a polygon, listed in consecutive order, find its area.
The first line contains an integer n~(3 \le n \le 50000), the number of vertices.
Each of the next lines contains two integers x_i and y_i~(-1000 \le x_i, y_i \le 1000) — the coordinates of the vertices of the polygon in consecutive order.
Print the area of the polygon with digits after the decimal point.
Example 1