Alice and Bob are playing a game with the following rules:
1. There are $21$ stones in a pile.
2. The players take turns removing either $1$, $2$, or $3$ stones from the pile each turn.
3. The player who removes the last stone wins the game.
Alice starts first. To ensure her victory, how many stones should Alice remove on her first turn? Let's think step by step and output the final answer within \boxed{}.
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