Five students Xiao Lin, Xiao Wang, Xiao Li, Xiao Zhang and Xiao Xu participated in a chess game together, and each of them had to play a game. So far, Xiao Lin has played 4 sets, Xiao Wang has played 3 sets, Xiao Li has played 2 sets, and Xiao Zhang has played 1 set. How many sets has Xiao Xu played.
小林、小王、小李、小张和小许五位同学一起参加象棋比赛，每两人都要比赛一盘．到现在为止，小林已赛4盘，小王已赛3盘，小李已赛2盘，小张已赛1盘．小许已赛了多少盘．
<result>2</result>