(double input=Convert.ToDouble(textBox1.Text)

{
private void button1_Click(object sender, EventArgs e)

    {
        double input....
        double radius= Math.PI*input*input;
        Graphics paper;
        paper = pictureBox1.CreateGraphics();
        Pen pen = new Pen(Color.Black);
        getCircle(paper, pen, (variables for center), radius);
    }
private void getCircle(Graphics drawingArea, Pen penToUse, int xPos, int yPos, double radius);

{
}

}

private void button1_Click(object sender, EventArgs e)
{
    int centerX;
    int centerY;
    int radius;

    if (!int.TryParse(textBox2.Text, out centerX))
        return;
    if (!int.TryParse(textBox3.Text, out centerY))
        return;
    if(! int.TryParse(textBox1.Text,out radius))
        return;

    Point center = new Point(centerX, centerY);

    Graphics paper;
    paper = pictureBox1.CreateGraphics();
    Pen pen = new Pen(Color.Black);
    getCircle(paper, pen, center, radius);

}

private void getCircle(Graphics drawingArea, Pen penToUse, Point center, int radius)
{
    Rectangle rect = new Rectangle(center.X-radius, center.Y-radius,radius*2,radius*2);
    drawingArea.DrawEllipse(penToUse,rect);
}