The school holds a grade football match. There are 10 classes in the sixth grade. There is a game between every two classes. How many games are there in total?
学校举行年级足球比赛，六年级有10个班，每两个班之间都要打一场比赛，一共要打多少场比赛．
<gadget id="calculator">10 - 1</gadget>
<output>9</output>

<gadget id="calculator">10 / 2</gadget>
<output>5</output>

<gadget id="calculator">9 * 5</gadget>
<output>45</output>

<result>45</result>