The school plants a tree every 3 meters on one side of the sidewalk in front of the teaching building. A total of 518 trees are planted from one end of the sidewalk to the other (both ends are planted). How many meters is the sidewalk in the school?
学校在教学楼前的人行道一边每隔3米栽一棵树，从人行道一端到另一端(两端都栽)共栽518棵树，学校的这人行道长多少米．
<gadget id="calculator">518 - 1</gadget>
<output>517</output>

<gadget id="calculator">3 * 517</gadget>
<output>1_551</output>

<result>1_551</result>