20 trees are planted every 4 meters, how many meters are there from the first tree to the 20th tree?
20棵树，每4米栽一棵，从第一棵到第20棵共有多少米．
<gadget id="calculator">20 - 1</gadget>
<output>19</output>

<gadget id="calculator">19 * 4</gadget>
<output>76</output>

<result>76</result>