5 classmates shake hands once between two people, how many times should they shake hands in total?
5个同学，每两个人之间握一次手，一共要握多少次．
<gadget id="calculator">5 - 1</gadget>
<output>4</output>

<gadget id="calculator">4 / 2</gadget>
<output>2</output>

<gadget id="calculator">5 * 2</gadget>
<output>10</output>

<result>10</result>