There are 6 classmates who meet again after a long absence. Each of them has to shake hands with other classmates once. How many times do they shake hands in total?
有6名同学久别重逢，每个人都要和其他同学握一次手，他们一共要握多少次手．
<gadget id="calculator">6 - 1</gadget>
<output>5</output>

<gadget id="calculator">5 / 2</gadget>
<output>5/2 = around 2.5</output>

<gadget id="calculator">6 * (5/2)</gadget>
<output>15</output>

<result>15</result>