The upper base of an isosceles trapezoid is 7 cm, the lower base is 12 cm, and the length of one waist is 6 cm. How many centimeters of iron wire are needed to enclose this isosceles trapezoid?
一个等腰梯形上底是7厘米，下底是12厘米，一条腰长6厘米，围成这个等腰梯形至少需要多少厘米的铁丝．
<gadget id="calculator">6 * 2</gadget>
<output>12</output>

<gadget id="calculator">7 + 12 + 12</gadget>
<output>31</output>

<result>31</result>