[SOLVED]GLSL 2x2 dithering mask problem

So I want to generate 2x2 dithering mask, simple checkers pattern. Here is my code:

It looks fine, but unfortunately, there are some ugly artifacts, that will twink and twitch if you move the camera even slightest.

Pics:
i.imgur.com/6Vf1T89.jpg
i.imgur.com/XPEqWTp.jpg

I’ve tried to change my expression a bit, shuffle summands, but result is always the same.

What is it? Float float precision error? Any ideas how to fix it?

Thank you very much. That’s looks more tidy, and actually work.

So you have G-buffer compression in your PBR thingy? I should probably check it out.