help in conditional field based on two other fields

[Sam]

Hello,

I'm currently creating a prototype to evaluate JIRA and ScriptRunner, and have some issues I need to learn about.
1- How do I set a "number field" to display values based on another field's value based on the condition  when updating an issue.

here field A and field B are number fields.
If field A > 0 & Field B <0

then field C = yes
2- Also , 

If field A < 0 & Field B >0

then field C = no

Thank you,