I want to put an input mask on my cells, so that it automatically spaces out the numbers form letters

E.g.

If I enter "192gr"
i want the mas to display it as " 192 gr" (with the space) each entry that I will enter will end with two letters (but i always forget to put the space in between them

How do I get this done?

A sample of my document which I'm using is here on Google Docs >> [https://docs.google.com/spreadsheet/...1F1MFNaZTdKNGc]