There are 4 classes in a tug-of-war competition, and every two classes must compete once. How many games will there be in total?
有4个班级进行拔河比赛，每两个班级必须比赛一场，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>