8 students need to shake hands once between each two, how many times do they need to shake hands?
8名同学每两人之间要握一次手，需要握多少次．
<gadget id="calculator">8 - 1</gadget>
<output>7</output>

<gadget id="calculator">8 / 2</gadget>
<output>4</output>

<gadget id="calculator">7 * 4</gadget>
<output>28</output>

<result>28</result>