There are 5 students, every two people shake hands once, how many handshakes in total.
有5位同学，每两个人握一次手，一共握多少次手．
<gadget id="calculator">5 - 1</gadget>
<output>4</output>

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

<gadget id="calculator">5 * 2</gadget>
<output>10</output>

<result>10</result>