12 football teams play a single round-robin match. It is known that any 4 teams are selected at a certain time, and two teams must have played against each other. So at least how many games have been played?
12支足球队进行单循环赛，已知某时刻任取4支队伍一定有两支队伍相互赛过，那么至少已经赛了多少场？
<gadget id="calculator">12 - 1</gadget>
<output>11</output>

<gadget id="calculator">11 / 2</gadget>
<output>11/2 = around 5.5</output>

<gadget id="calculator">12 * (11/2)</gadget>
<output>66</output>

<result>66</result>