Rule / Trigger Help

[Nescio]

I have a table, let's call it table 1, with two integer columns in it. I would like to be able to efficiently generate the entry for column 2 based on the entry in column 1. Basically I would like a private sequence for every entry in column 1. So the final result would look something like this.

I've been poking around with rules, functions and triggers for a while but I haven't been having much luck. Thanks in advance for any advice.