Help for an automation exit of a zone: from a gps coordinates (device_tracker) and a radius (a number)

Hi I need to do an automation that when a button is presses the GPS coordinates of the object (a device_tracker) are stored , also with a presettable number (a radius) , when the oblect exits the area of the radius it triggers an alarn.

How can I achieve this? The ZONE: can be dinamic and created upon a button press? Or other solutions?

Thanks