There are 5 cities, and a highway is built between every two cities, how many highways can be built.
有5个城市，每两个城市之间修一条高速公路，可以修多少条高速公路．
<gadget id="calculator">5 - 1</gadget>
<output>4</output>

<gadget id="calculator">5 / 2</gadget>
<output>5/2 = around 2.5</output>

<gadget id="calculator">4 * (5/2)</gadget>
<output>10</output>

<result>10</result>