There are 21 chess pieces that are taken by two people in turn, and each person can only take one or three pieces at a time, and whoever gets the last one wins. A takes first, how many can be taken first to win.
有21个棋子两人轮取，每人每次只能取1个或3个，谁取到最后一个谁就获胜．甲先取，先取多少个才能取胜．
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