classes play volleyball, and every two classes have a game. Ask: How many games are there in total?
4个班比赛打排球，每两个班都要比赛一场，问：一共要比赛多少场？
<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>