During the holiday, 8 classmates shook hands one by one to say goodbye. Every two people shook hands once, and only once. How many times did they shake hands in total?
8个同学放假时，一一握手告别，每两人都握一次，而且只握一次手，共握了多少次．
<gadget id="calculator">7 + 6 + 5 + 4 + 3 + 2 + 1</gadget>
<output>28</output>

<result>28</result>