There are 20 people, and every two people shake hands once, how many times should they shake hands in total?
有20个人，每两人握一次手，一共要握多少次手．
<gadget id="calculator">20 - 1</gadget>
<output>19</output>

<gadget id="calculator">20 / 2</gadget>
<output>10</output>

<gadget id="calculator">19 * 10</gadget>
<output>190</output>

<result>190</result>