A train runs from Shanghai to Nanjing and passes through 6 stations on the way. How many different tickets should be prepared for this train?
一列火车从上海开到南京，中途要经过6个站，这列火车要准备多少种不同的车票．
<gadget id="calculator">7 + 6 + 5 + 4 + 3 + 2 + 1</gadget>
<output>28</output>

<result>28</result>