I want a check box to add a number to another box

[Tyler26022222bim4]

I'm working on a character sheet and im working on making it auto fill. Im trying to figure out how when i click the bubble to the left of athletics, it will add the bottom proficency bonus to the athletic skill but always add the top str bonus even when the bubble isnt checked. 

[Tyler26022222bim4]

I got it to work using this code

if(this.getField("athc").value != "Off"){
event.value = (this.getField("strmod").value) + (this.getField("Prob").value);}
else
event.value = (this.getField("strmod").value);