The new semester begins, and 10 classmates meet each other. If every two classmates shake hands once, how many times will they shake hands in total?
新学期开学了，10个同学见面，如果每两个同学都握一次手，那么一共要握多少次手．
<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>