4 people A, B, C, and D meet each other, and every 2 people shake hands once, how many handshakes should they have in total?
甲、乙、丙、丁4个人见面，每2个人握手一次，一共要握多少次手．
<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>