If I need to go from the airport to the downtown area on day 1 and then return to the airport on day 3 (with not much travel being done on day 2), what is the very cheapest ticket that I can buy? Would it be better to combine separate Innenraum tickets with separate tickets between the airport and the outer border of the Innenraum? Or do I have to spring for two Gesamtnetz tickets @ €13 each?