There are 4 football teams in total, and every two teams play one game, how many games will be played.
共有4支足球队，每两队踢一场，要踢多少场．
<gadget id="calculator">4 - 1</gadget>
<output>3</output>

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

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

<result>6</result>