There are 10 classmates shaking hands to say goodbye, every two classmates shake hands once, how many times they shook hands in total.
有10个同学握手话别，每两个同学握一次手，他们一共握了多少次手．
<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>