There are 10 people who get together and shake hands with each other, and each person should shake hands with the other 9 people once, without repetition or omission. How many times to shake hands in total.
有10人聚在一起，相互握手致意，每人都要和其余9人握一次手，不重复、不遗漏．一共要握手多少次．
<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>