4 classmates shake hands, every two people shake hands, how many times do they need to shake hands?
4个同学握手，每两个人握一次，需握多少次．
<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>