When 7 children meet, every two children shake hands once, how many times do they shake hands in total?
7个小朋友见面，每两个小朋友握一次手，他们共握多少次手．
<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>