I have a visual flow where I am adding screen choice feilds together in a formula.  It allows me to add them together as long as the screen choices are answered in the flow. The flow skips questions depending on answers though.  How do I get a flow formula to add all of my screen choices together even if a screen choice is bypassed?

I have a visual flow where I am adding screen choice feilds together in a formula.  It allows me to add them together as long as the screen choices are answered in the flow. The flow skips questions depending on answers though.  How do I get a flow formula to add all of my screen choices together even if a screen choice is bypassed?