10 students participate in the "You ask me to answer" interesting knowledge competition. It is stipulated that every two students have to compete once, and how many competitions are required in total.
10名同学进行"你问我答"趣味知识比赛，规定每两名同学都要赛一次，共需比赛多少次．
<gadget id="calculator">10 * 9</gadget>
<output>90</output>

<gadget id="calculator">90 / 2</gadget>
<output>45</output>

<result>45</result>