The area of a polygon#851
A simple polygon is given on the plane by the integer coordinates of its vertices in the Cartesian coordinate system. The polygon is simple: its sides do not intersect or touch each other except at adjacent vertices.
Find the area of the given polygon.
The first line contains an integer (3 \leq n \leq 50000) --- the number of vertices of the polygon. Each of the next lines contains two integers --- the coordinates of a vertex. The vertices are listed in order along the polygon. If you connect all the points in this order and also connect the first and last point, you obtain the polygon.
It is guaranteed that all coordinates are integers and their absolute values do not exceed .
Print a single number --- the area of the polygon. Your answer should be rounded to the nearest number with one digit after the decimal point.
Beispiel 1