1 entry, 1 exit, 1000 doors

A group of friends explore the countryside and find an interesting old looking builiding. Being the curious teens they are, they decide to go inside it. They are soon trapped when the one entry to the building shuts closed. The good thing is that there's another exit. The bad thing is that they need to find it before the old, crumbling building collapses on then before they have the chance to get out. Now the really bad news is that they have to find the only exit using 1000 different staircases that lead to 1000 different doors. One of them is the exit. Will they escape on time?