15. There is a two-player game, with 13 Go stones as the game props. The player who goes first is determined by drawing lots or similar methods, referred to as the first player, and the other player is referred to as 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's turn, the second player begins to take Go stones according to the same rules: both players take turns picking up stones until all the stones are taken. The player who takes the last stone wins. The first player has a winning strategy in this game. To win, the number of Go stones the first player should leave for the opponent from the first round to the win should be $\qquad$.
$12,9,6,3$