Transparent Circle Painting

bool b = false;
        int x1, y1 = 0;
        private void pictureBox1_MouseMove(object sender, MouseEventArgs e)
        {
            if (b == true)
            {
                SolidBrush sb = new SolidBrush(Color.White);
                Bitmap bi = new Bitmap(Width, Height);
                Graphics g = Graphics.FromImage(bi);
                int xx = e.X - x1;
                int yy = e.Y - y1;
                g.SmoothingMode = System.Drawing.Drawing2D.SmoothingMode.AntiAlias;
                g.DrawEllipse(new Pen(sb), new Rectangle(x1, y1, xx, yy));
                sb.Color = Color.FromArgb(150, 255, 0, 0);
                g.FillEllipse(sb, new Rectangle(x1, y1, xx, yy));
                pictureBox1.Image = bi;

            }
        }

        private void pictureBox1_MouseUp(object sender, MouseEventArgs e)
        {
            b = false;
        }

        private void pictureBox1_MouseDown(object sender, MouseEventArgs e)
        {
            b = true;
            x1 = e.X;
            y1 = e.Y;
        }