There are four classes in the third grade of Experimental Primary School. They have a basketball round-robin game. (Every two classes have one game) How many games do they have to play?
实验小学三年级有四个班，他们进行篮球循环比赛，(每2班比赛一场)他们要比赛多少场．
<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>