There are 4 classmates who met each other, and each of them shook hands once, how many times in total.
有4个同学见了面，每2人都握一次手，一共握了多少次．
<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>