people meet at a meeting, and every two people shake hands once, how many times should they shake hands in total?
5人在一次会议上见面，每两人握一次手，一共要握手多少次．
<gadget id="calculator">4 / 2</gadget>
<output>2</output>

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

<result>10</result>