How many days does a commodity fair last from December 20th to January 28th?
某商品展销会展销时间是从12月20日到元月28日，为期多少天．
<gadget id="calculator">31 - 20</gadget>
<output>11</output>

<gadget id="calculator">11 + 1 + 28</gadget>
<output>40</output>

<result>40</result>