4 good friends meet each other, every two people shake hands once, how many times in total.
4个好朋友见面，每两个人握一次手，一共握多少次手．
<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>