The school held a table tennis selection competition. Each player had to play against all other players. A total of 78 games were played. How many people participated in the selection competition?
学校进行乒乓球选拔赛，每个参赛选手都要和其他所有选手赛一场，一共进行了78场比赛，有多少人参加了选拔赛．
<result>13</result>