How to get the top parent Subject?

Hi,

I have a subject 1 that is the parent of subject 2. Subject 2 is parent of subject 3.

On opportunities I have added a field "Subject", the value is subject 3. On change of my new field I would like to populate Another new field "Top subject" with the top parent (subject 1).

I know that I must have a Jscript that triggers on the OnChange event. How do I get the top parent subject?

Any help is really appriciated.