There are 8 students who shake hands to say goodbye, every two people shake hands once, how many times did they shake hands in total?
有8个同学握手告别，每两个人都握一次手，一共握了多少次手．
<gadget id="calculator">7 + 6 + 5 + 4 + 3 + 2 + 1</gadget>
<output>28</output>

<result>28</result>