10 old friends got together, and each of them shook hands once, how many times did they shake hands in total?
10位老朋友聚会，每两人之间都握了一次手，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>