6 classmates shake hands, every two shake hands, how many times should they shake hands in total?
6个同学握手，每两个握一次，一共要握多少次手．
<gadget id="calculator">6 - 1</gadget>
<output>5</output>

<gadget id="calculator">6 / 2</gadget>
<output>3</output>

<gadget id="calculator">5 * 3</gadget>
<output>15</output>

<result>15</result>