A game involves two players, Alice and Bob, who take turns removing stones from a single pile. On each turn, a player can remove 1, 2, or 3 stones from the pile. The player who removes the last stone wins. Initially, there are 21 stones in the pile. If Alice goes first and both players play optimally, how many stones should Alice remove on her first turn to guarantee a win? Let's think step by step and output the final answer within \boxed{}.
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