There are 20 people gathered together, shaking hands with each other, each person has to shake hands with the other 19 people, no repetition, no omission, how many handshakes in total.
有20人聚在一起，相互握手致意，每人都要和其余19人握一次手，不重复，不遗漏，一共要握手多少次．
<gadget id="calculator">20 - 1</gadget>
<output>19</output>

<gadget id="calculator">19 / 2</gadget>
<output>19/2 = around 9.5</output>

<gadget id="calculator">20 * (19/2)</gadget>
<output>190</output>

<result>190</result>