students participate in the competition, every two students have to play one game, how many games are there in total.
9位同学参加比赛，每两位同学都要比赛一场，一共要比赛多少场．
<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>