Xiao Ming wants to put 210 chess pieces in several boxes. Put 1 piece in the first box, 2 pieces in the second box, 3 pieces in the third box, ..., and so on, until the chess pieces are just finished. So how many boxes did Xiao Ming use?
小明想把210枚棋子放在若干个盒子里．按第一个盒子里放1枚，第2个盒子里放2枚，第3个盒子里放3枚，…，这样下去，最后刚好将棋子放完．那么小明用了多少个盒子呢？
<result>20</result>