A total of 10 people participated in a meeting. At the end of the meeting, they shook hands and said goodbye. Every two people shook hands once. How many times did they shake hands in total?
一次会议共10个人参加，会议结束时，握手告别，每两人都握一次手，一共握多少次手．
<gadget id="calculator">10 - 1</gadget>
<output>9</output>

<gadget id="calculator">9 / 2</gadget>
<output>9/2 = around 4.5</output>

<gadget id="calculator">10 * (9/2)</gadget>
<output>45</output>

<result>45</result>