In a game, players take turns picking a number of objects from a pile. On each turn, a player can take any positive number of objects, but not more than the number taken by the previous player. The player who picks the last object wins. Initially, there are 21 objects in the pile. If the first player uses an optimal strategy, how many objects should they take on their first turn to guarantee a win? Let's think step by step and output the final answer within \boxed{}.
user