typedef struct {
   double x,y;
} Point;

double PolygonArea(Point *polygon,int N)
{
   int i,j;
   double area = 0;

   for (i=0;i<N;i++) {
      j = (i + 1) % N;
      area += polygon[i].x * polygon[j].y;
      area -= polygon[i].y * polygon[j].x;
   }

   area /= 2;
   return(area < 0 ? -area : area);
}