21 children line up in a row, with an interval of 2 meters between every two children. How many meters is the distance from the first child to the last child?
21个小朋友排成一行，每两个小朋友之间间隔2米．第一个小朋友到最后一个小朋友相距多少米．
<gadget id="calculator">21 - 1</gadget>
<output>20</output>

<gadget id="calculator">20 * 2</gadget>
<output>40</output>

<result>40</result>