15. There is a two-player game, with 22 Go stones as the game props. The player who goes first is determined by drawing lots or guessing, and is called the first player, while the other is called the second player. The game rules are as follows: The first player must choose to take 1 or 2 Go stones; after the first player completes their turn, the second player begins to take Go stones according to the same rules; both players take turns until all the stones are taken. The player who takes the last stone wins. The first player has a winning strategy in this game. If the first player wants to win, the number of Go stones they should leave for their opponent from the first round to the end of the game is $\qquad$
【Analysis】 $21, 18, 15, 12, 9, 6, 3, 0$