A trapezoidal billboard with an upper base of 12 meters, a lower base of 18 meters and a height of 4 meters. How many square meters are painted on the front and back sides of this billboard?
一梯形广告牌，上底12米，下底18米，高4米．油漆这块广告牌正反两面一共油漆多少平方米？
<gadget id="calculator">12 + 18</gadget>
<output>30</output>

<gadget id="calculator">4 / 2</gadget>
<output>2</output>

<gadget id="calculator">30 * 2 * 2</gadget>
<output>120</output>

<result>120</result>