Put colorful flags on both sides of a runway, insert one side every 5 meters, and insert 20 sides in total. How long is this runway in meters?
在一条跑道两边插彩旗，每隔5米插-面，共插了20面，这条跑道长多少米．
<gadget id="calculator">20 / 2</gadget>
<output>10</output>

<gadget id="calculator">10 - 1</gadget>
<output>9</output>

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

<result>45</result>