What is the area of a parallelogram whose height is 4 decimeters and its base is 1 decimeter shorter?
一个平行四边形的高4分米，比它的底短1分米，它的面积是多少．
<gadget id="calculator">4 + 1</gadget>
<output>5</output>

<gadget id="calculator">5 * 4</gadget>
<output>20</output>

<result>20</result>