draw polygon using mouse up and down

private Graphics grap;
private List<point> polygonP=new List<point>();
private void panel1_MouseDown(object sender, MouseEventArgs e)
       {
           x1 = e.X;
           y1 = e.Y;
           polygonP.Add(new Point(e.X, e.Y));
       }
private void panel1_MouseUp(object sender, MouseEventArgs e)
        {
            Graphics grap = panel1.CreateGraphics();
            Pen mypen = new Pen(colour,linewidth);
             polygonP.Add(new Point(e.X,e.Y));
                        if(polygonP.Count>1)
                        {
                            grap.DrawLine(mypen,polygonP[polygonP.Count-2],polygonP[polygonP.Count-1]);
                        }
                        if (polygonP.Count > 2)  *****
                        {
                            grap.DrawLine(mypen, polygonP[polygonP.Count - 1], polygonP[0]);
                            polygonP.Clear();
                        }