There is a rope stretched in a straight line. The points at 20 equal divisions are marked with red marks; the points at 21 equal divisions are marked with blue marks. The length between the red mark and the blue mark is 2 cm at the shortest point. What is the length of the rope in cm?
有一条拉长成直线的绳子．将其20等分时的点，涂上红色记号；21等分时的点，涂上蓝色记号．红色记号与蓝色记号之间的长度，最短处为2厘米，则此绳子的长为多少厘米．
<gadget id="calculator">20 * 21</gadget>
<output>420</output>

<gadget id="calculator">420 * 2</gadget>
<output>840</output>

<result>840</result>