There are 9 students playing badminton, and every two students play a game, how many games have been played in total?
有9名同学羽毛球比赛，每两名同学都进行一场比赛，共经行了多少场比赛．
<gadget id="calculator">9 - 1</gadget>
<output>8</output>

<gadget id="calculator">8 / 2</gadget>
<output>4</output>

<gadget id="calculator">9 * 4</gadget>
<output>36</output>

<result>36</result>