'width' and 'height' are not members of 'Screen'?

I follow the tutorial to execute

function OnGUI () {
	GUI.Box (Rect (0,0,100,50), "Top-left");
	GUI.Box (Rect (Screen.width - 100,0,100,50), "Top-right");
	GUI.Box (Rect (0,Screen.height - 50,100,50), "Bottom-left");
	GUI.Box (Rect (Screen.width - 100,Screen.height - 50,100,50), "Bottom-right");

but it comes to
BCE0019: 'width' is not a member of 'Screen'. BCE0019: 'height' is not a member of 'Screen'.
and due to the script reference, I think that they are members of screen.

How should I fix it? Thx in advance.

You named one of your own classes / scripts “Screen”. rename it! Never use built-in classnames for your own stuff.