Plant trees on one side of a path, planting one tree every 4 meters (both ends are planted), exactly 20 trees are planted. How many meters is this path?
在一条小路的一侧植树，每隔4米植1棵，(两端都植)，正好植了20棵．这条小路有多少米长．
<gadget id="calculator">20 - 1</gadget>
<output>19</output>

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

<result>76</result>