AE EXPRESSION - Random Position

I'm starting to learn expression for after effects and I have a question that I couldn't find the answer to on the internet forums.

I have this grid (4x4 in a 1920x1920 area) where this elements have random positions that follow a seed number on a slider in a null.

I uploaded an image to show what I am talking about. One of the images is the expression that I’m using and the other the grid and elements. 

In all seed numbers the elements overlap (as shown in the picture), is there any way to prevent them from doing this? Any expression?

Observations:

Small elements have an area of 480x480px and large elements have an area of 960x960px.

My after effects is in Portuguese (I’m Brazilian) so “Controle Deslizante” is “Slider”, “Posição” is “Position” and “Controle” is “Control”