Add days to date Formula

I have this formula which adds days from Date Field A: addDays($date_a_c,259) . It gives me a date calced off of Date A and puts it to Date Field B. I need a formula which does this, unless Date Field D has a date, in which it should calc from field D.

I cant seem to figure it out. Can I get some pointers to which formula to use there? My rough idea is something like this: addDays($date_a_c, or(addDays($date_d_c,259)) , but of course this doesnt work.