An isosceles trapezoidal vegetable field, the upper base is 12 meters long, the lower base is 20 meters long, and the waist is 8 meters shorter than the lower base. What is the perimeter of this vegetable plot in meters?
一块等腰梯形菜地，它的上底长12米，下底长20米，腰比下底短8米，这块菜地的周长是多少米？
<gadget id="calculator">20 - 8</gadget>
<output>12</output>

<gadget id="calculator">12 * 2</gadget>
<output>24</output>

<gadget id="calculator">24 + 12 + 20</gadget>
<output>56</output>

<result>56</result>