Four classes in the sixth grade participate in the tug-of-war competition. Every two classes have a competition. How many games are there in total?
六年级4个班参加拔河比赛，每两个班都要进行一次比赛，共有多少场比赛．
<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>