Trees are to be planted on one side of the road and both ends. It was originally planned to plant a tree every 4 meters, and a total of 21 trees will be planted. Now it is changed to plant a tree every 5 meters. How many saplings are needed now?
公路的一侧要植树，两端都要栽，原计划每隔4米种一棵树，一共要种21棵树，现在改为每隔5米种一棵树，现在要多少棵树苗．
<gadget id="calculator">21 - 1</gadget>
<output>20</output>

<gadget id="calculator">20 * 4</gadget>
<output>80</output>

<gadget id="calculator">80 / 5</gadget>
<output>16</output>

<gadget id="calculator">16 + 1</gadget>
<output>17</output>

<result>17</result>