There are 7 children standing in a row, and every two people shake hands once, how many times do they need to shake hands in total?
有7个小朋友站成一排，每两人握一次手，他们一共需要握多少次手．
<gadget id="calculator">7 - 1</gadget>
<output>6</output>

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

<gadget id="calculator">7 * 3</gadget>
<output>21</output>

<result>21</result>