If 5 people meet each other and shake hands once, how many times should they shake hands in total?
5个人见面相互握手一次，一共要握多少次．
<gadget id="calculator">4 / 2</gadget>
<output>2</output>

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

<result>10</result>