There are 10 football teams participating in a game. If every two teams have to play one game, how many games will they play in total?
10个足球队参加比赛，如果每两个队都要赛一场，他们一共要比赛多少场．
<gadget id="calculator">10 - 1</gadget>
<output>9</output>

<gadget id="calculator">9 / 2</gadget>
<output>9/2 = around 4.5</output>

<gadget id="calculator">10 * (9/2)</gadget>
<output>45</output>

<result>45</result>