4 good friends met each other, and shook hands once every 2 people, how many handshakes are needed in total.
4位好朋友见了面，每2人之间握了一次手，一共需要握多少次．
<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>