There are 5 classmates together, and every two of them have to shake hands once, how many times in total.
有5个同学在一起，他们每两人要握手一次，一共握手多少次．
<gadget id="calculator">5 - 1</gadget>
<output>4</output>

<gadget id="calculator">5 / 2</gadget>
<output>5/2 = around 2.5</output>

<gadget id="calculator">4 * (5/2)</gadget>
<output>10</output>

<result>10</result>