A, B, C, D and Xiao Wang are playing table tennis. Each of them has to play one set. So far, A has played 4 sets, B has played 3 sets, C has played 2 sets, and D has played 1 set. How much has Xiao Wang played? plate.
甲乙丙丁和小王比赛乒乓球，每两人都要比赛一盘，到目前为止，甲赛了4盘，乙已赛3盘，丙已赛2盘，丁比赛1盘，小王赛了多少盘．
<result>2</result>