Script to sum two Number fields and display result in third field?

[kdickason]

This should be very similar for you advanced scripters:  I have two Number fields ("Field A", "Field B") that I want to sum in a "Total" field.  I have script runner.  Can someone post for me the entire script that I could use to make this work?  I'm a weak scripter and need the entire script, not just excerpts.  Thanks you so much to anyone who can help.

This is similar to the question posted here, but I need someone to post the complete working script for me, which doesn't seem to have occurred in this entry:  https://community.atlassian.com/t5/Adaptavist-questions/SUM-the-values-of-2-Number-Custom-Fields-in-a-3rd-Custom/qaq-p/1248782#U2122588