Among the 12 cities, there is a direct air route between every two cities. How many air routes are there in total?
在12个城市中，每两个城市之间都有一条直达的航空线，一共有多少条航空线？
<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>