There are 10 football teams participating in a game, and every two teams will play one game. How many games will there be in total?
10个足球队参加比赛，每两个队都要赛一场，一共要赛多少场．
<gadget id="calculator">10 - 1</gadget>
<output>9</output>

<gadget id="calculator">10 / 2</gadget>
<output>5</output>

<gadget id="calculator">9 * 5</gadget>
<output>45</output>

<result>45</result>