5 children shake each other's hands once every 2 people, how many handshakes are needed in total.
5个小朋友每2人相互握一次手，共需握多少次．
<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>