When 4 people meet each other, each of them shakes hands once, how many times in total.
4个人见面，每两人之间握一次手，共握了多少次手．
<gadget id="calculator">4 - 1</gadget>
<output>3</output>

<gadget id="calculator">4 / 2</gadget>
<output>2</output>

<gadget id="calculator">3 * 2</gadget>
<output>6</output>

<result>6</result>