There are 5 classmates who go to a party, and every two people shake hands once, how many times have they shake hands in total?
有5个同学去参加聚会，每两个人之间握一次手，一共握了多少次手．
<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>