C#中如何画圆?只看到用内置矩形画圆?那假如我要画一个半径是100的圆应该怎么写呢?

Point center = new Point(20,20);
Graphics paper = pictureBoxDisplay.CreateGraphics();
paper.Clear(this.BackColor);
SolidBrush aBrush = new SolidBrush(Color.Green);
Pen blackPen = new Pen(Color.Black,1);
int r = int.Parse(textBox1.Text);
Rectangle bar = new Rectangle(center.X-r,center.Y-r,center.X+r,center.Y+r);
paper.FillEllipse(aBrush,bar);
paper.DrawEllipse(blackPen,bar);
bar.Y += 80;
aBrush.Color = Color.Yellow;
paper.FillEllipse(aBrush,bar);
paper.DrawEllipse(blackPen,bar);
bar.Y += 80;
aBrush.Color = Color.Red;
paper.FillEllipse(aBrush,bar);
paper.DrawEllipse(blackPen,bar);