C# converting Input.Mouseposition.y to to "top" argument of GUI.Box

Note: There are some syntax errors in the code i wrote as example, this is just pseudo code, I posted my own code from my script under it.
This question may seem long, but its actually fairly short, I just wrote alot of stuff to put you guys in context.

Hi guys, I’m coding a box selection system, and I want to display my box on screen by a GUI.Box.
However, the arguments for Input.mouseposition start from the bottom left of the screen, E.G:

Vector2 MousePos = Input.MousePosition;
If your mouse is at the bottom left of screen it will give:
MousePos = (0,0)

And the Arguments for GUI.Box are:

(Left, Top, Width, Height)

So if you enter your MousePos of (0,0) (bottom left) in there:

GUI.Box(new GUI.Box(MousePos.x (0), MousePos.y (0), 100, 100));

This will start the box from the top left instead of the bottom left, since the origin of the box starts from the top of the screen and the origin of input.mouseposition starts from the bottom of the screen!

Add all of this together, if my MousePos.y is 100, the box will not start 100 units from the bottom of the screen, but 100 units from the top of the screen, mirroring the Y position of the mouse on the horizontal axis of the screen!

My question is: How can I manage to get the “Top” argument from my GUI.Box to match the Input.MousePosition.y on my game screen?

Here is my code:

	void Update () {
			if(LeftMouseDown == false){
				LeftMouseDown = true;
				mouseOriginalPos = Input.mousePosition;
			mouseCurrentPos = Input.mousePosition;
			SelectionBox = new Rect(Mathf.Min(mouseOriginalPos.x, mouseCurrentPos.x), Mathf.Min(mouseOriginalPos.y, mouseCurrentPos.y),
									Mathf.Abs(mouseOriginalPos.x - mouseCurrentPos.x), Mathf.Abs(mouseOriginalPos.y - mouseCurrentPos.y));

	void OnGUI(){
		if(LeftMouseDown == true)
			GUI.Box(new Rect(mouseOriginalPos.x, mouseOriginalPos.y, mouseCurrentPos.x - mouseOriginalPos.x, mouseOriginalPos.y - mouseCurrentPos.y), "");	

the SelectionBox Rect variable is in no way affecting my GUI.Box

LeftMouseDown is a bool variable that I reset to false once i release my mouse button.
Notice that I directly enter mouseOriginalPos.y as the “Top” argument of the box, resulting in this “mirror” effect of the mouse position and top of the box.

Thank you very much for anyone who can help!

You need to do all of your mouse calculations in the GUI if you want to use it in OnGUI. You can get the mouse presses and the current position of the mouse from Event.current. The position of the mouse in Input is not relevant to the position of things in OnGUI - but the Event’s mouse positions are.