4 children, every two shake hands, how many times can they shake hands in total.
4个小朋友，每两个握一次手，一共可以握多少次．
<gadget id="calculator">4 - 1</gadget>
<output>3</output>

<gadget id="calculator">4 / 2</gadget>
<output>2</output>

<gadget id="calculator">3 * 2</gadget>
<output>6</output>

<result>6</result>