6 children attend a party, every 2 children have to shake hands once, how many handshakes are needed in total.
6个小朋友参加一次聚会，每2位小朋友都要握一次手，一共要握多少次手．
<gadget id="calculator">6 - 1</gadget>
<output>5</output>

<gadget id="calculator">6 / 2</gadget>
<output>3</output>

<gadget id="calculator">5 * 3</gadget>
<output>15</output>

<result>15</result>