6 friends who have been separated for a long time get together, and every 2 people shake hands once, how many times do they shake hands in total?
6个分别很久的朋友聚在一起，每2个人握一次手，他们一共要握多少次手．
<gadget id="calculator">6 - 1</gadget>
<output>5</output>

<gadget id="calculator">5 / 2</gadget>
<output>5/2 = around 2.5</output>

<gadget id="calculator">6 * (5/2)</gadget>
<output>15</output>

<result>15</result>