If 10 friends shake hands once when they meet each other, how many times will the 10 people shake hands in total?
10个朋友聚会，见面时如果每两人都要握一次手，那么，10个人共要握手多少次．
<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>