   Lua arithmetic
  I'm not  100% sure if this is the right area to post this, but I'm trying to come up with an equation  in Lua.The problem is that I want a circle to appear randomly within a range of numbers. The idea being that it reappears elsewhere on screen randomly, but I want the whole button to be on screen. I don 't have any part of the image to be cropped off by the screen. Here's the relevant code that I came up with:randomButton = display.newCircle(0, 0, 40)-- Makes the circle red (R, G, B)randomButton:setFillColor(200, 0, 0)-- Gets the height and width of the screen on the current platformlocal _H = display.contentHeight;  local _W = display.contentWidth -- Puts the randomButton at the center of the screen to startrandomButton.x = _W/2; randomButton.y = _H/2; -- Sets a reference point point for the randomButton in the middle of the circlerandomButton:setReferencePoint(display.CenterReferencePoint)randomButton:setFillColor (math.random(255) , math.random(255), math.random(255))randomButton.x = math.random(_W) randomButton.y = math.random(_H)Now I've figured out that the most I want the width to be between 40 and 280. I also want to the height to be between 40 and 440. So what equation would I have to put in on the last line of code to get that result?   
  The closest to the origin the button can be will be the circle radius. The furthest it can be is the screen size minus the radius.randomButton.x =  math.random(circleRadius, _H - circleRadius) randomButton.y =  math.random(circleRadius, _V - circleRadius   