How do I do calculation outside a optional sections

[Drikus Mouton]

I have 3 optional sections. Each one have a number that is based on certain
question in that optional section. Now I want to do calculation outside the
section based on the value inside the optional section. Ex.
Section A contains the value 5
Section B contains the value 6
Section C contains the value 4
If just A is selected the number must be 5
If A & B is selected the number must be 11
If all are selected the number must be 15.
I get always "NaN" if I don't insert all the optional sections.