There are 4 students shaking hands, every two students shake hands once, how many times do they shake hands in total?
有4名同学握手，每两名要握一次手，他们一共要握手多少次．
<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>