Students in 4 classes hold a singing competition, and every two classes must have one competition. How many competitions will there be in total?
4个班的同学举行歌唱比赛，每两个班必须赛一场，一共要进行多少场比赛．
<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>