A simple question about C#

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C#

We have a circle moving on a screen. How to find the lowest height that the circle goes on the screen
For example, on a 200 x 200 pixel screen, we have a circle with a diameter of 10 pixels. This circle is randomly moving on this screen. How can I find out to what height the lowest part of this circle has gone down, for example, to show that this circle has not gone below the height of 35 pixels?

What I have tried:

 CircleF[] circles = CvInvoke.HoughCircles(grayFrame, HoughModes.Gradient, 2.0, 20.0, 100.0, 45.0, 34, 39);
   foreach (CircleF circle in circles)
   {
    CvInvoke.Circle(fieldMask2, Point.Round(circle.Center), , 
    (int)circle.Radius, new Bgr(Color.White).MCvScalar, 1);
}