4 students shake hands with each other, how many times should they shake hands in total?
4个同学互相握手，每两人握一次，一共要握多少次．
<gadget id="calculator">4 - 1</gadget>
<output>3</output>

<gadget id="calculator">3 / 2</gadget>
<output>3/2 = around 1.5</output>

<gadget id="calculator">4 * (3/2)</gadget>
<output>6</output>

<result>6</result>