There are 12 student gatherings, every two people shake hands once, how many times should they shake hands in total.
有12个学生聚会，每两个人之间握一次手，一共要握多少次手．
<gadget id="calculator">12 - 1</gadget>
<output>11</output>

<gadget id="calculator">11 / 2</gadget>
<output>11/2 = around 5.5</output>

<gadget id="calculator">12 * (11/2)</gadget>
<output>66</output>

<result>66</result>