The 6 classes of the third grade have a tug-of-war competition, and every two classes will compete in one game. How many games will there be in total?
三年级6个班进行拔河比赛，每两个班级比一场，一共要比多少场．
<gadget id="calculator">6 - 1</gadget>
<output>5</output>

<gadget id="calculator">6 / 2</gadget>
<output>3</output>

<gadget id="calculator">5 * 3</gadget>
<output>15</output>

<result>15</result>