There are 10 people who just met. If every two people shake hands once, how many handshakes are needed in total?
有10人刚见面．如果每两人握一次手，一共需要握多少次手．
<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>