public CircleS(double x, double y, double r)
        {
            if (r < 0)
            {
                throw new ArgumentException("r >= 0!!");
            }

            center = new PointS(x, y);
            rad    = r;
        }
 static public double Distance(PointS first, PointS second)
 {
     return(Math.Sqrt((first.x - second.x) * (first.x - second.x) + (first.y - second.y) * (first.y - second.y)));
 }