import java.util.Arrays;

public class Polygon
{

    private int ncorners = 0;
    private Point[] points;
   /**
      Constructs a Polygon object with no corners
   */
   public Polygon()
   {
       this.points = new Point[0];
       this.ncorners = 0;
   }

   /**
      Adds a point to the list.
      @param p the point to add
   */
   public void add(Point p)
   {
       points = addToArray(points, p);
       ncorners++;
   }

   /**
      Computes the area of a polygon.
      @return area of a polygon
   */
   public double getArea()
   {
      if (ncorners < 3) {
          return 0;
      }
      else if (ncorners == 3) {
          return getTriangleSize(points);
      }else {
          Point startingPoint = points[0];
          Point toDelete =  points[1];
          Point toDraw = points[2];

          points = removeFromArray(points, toDelete);
          Polygon p = new Polygon();
          for(Point a : points) {
              p.add(a);
          }

          return getTriangleSize(new Point[]{startingPoint, toDelete, toDraw}) + p.getArea();
      }
   }

   private double getTriangleSize(Point[] pointArray) {

       //Just making the triangle calculation algorithm easier to type.
       double x1 = pointArray[0].getX();
       double y1 = pointArray[0].getY();
       double x2 = pointArray[1].getX();
       double y2 = pointArray[1].getY();
       double x3 = pointArray[2].getX();
       double y3 = pointArray[2].getY();

       double answer = ((x1*y2) + (x2*y3) + (x3*y1) - (y1*x2) - (y2*x3) - (y3*x1)) / 2;
       if(answer < 0) answer = answer * (-1);
       return answer;
   }

   private Point[] addToArray(Point[] array, Point newPoint) {
       Point[] point = new Point[array.length + 1];
       for(int i = 0 ; i < array.length; i++) {
           point[i] = array[i];
       }
       point[point.length-1] = newPoint;
       return point;
   }

   private Point[] removeFromArray(Point[] array, Point toRemove) {
       int i, j;
       for (i = j = 0; j < array.length; j++) if (!(toRemove ==array[j])) array[i++] = array[j];
       return Arrays.copyOf(array, i);
   }

}