There is a endless binary tree, just like the image below. The big boss is in the root, you and your teammate, were transmitted to two nodes -- Nth node and Mth node. In this tree, you can only go up to arrive the root, that means you can't retreat any steps. Now your teammate sent a buff up to root, the buff will go step by step. You want to get that buff to defeat the boss, so you can wait to get this buff, or you can move to catch it, all you should do is to get the buff as quickly as possible, at last, you should tell where you got the buff.
4 10 4 9