4 classes have a basketball game, every two classes have a game, how many games will there be in total?
4个班级进行篮球赛，每两个班都要比赛一场，一共要比多少场．
<gadget id="calculator">4 - 1</gadget>
<output>3</output>

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

<gadget id="calculator">3 * 2</gadget>
<output>6</output>

<result>6</result>