What is the perimeter of an isosceles trapezoid with an upper base of 6 cm, a lower base of 7 cm, and a waist of 4 cm?
一个上底为6厘米，下底为7厘米，腰为4厘米的等腰梯形，它的周长是多少厘米．
<gadget id="calculator">4 * 2</gadget>
<output>8</output>

<gadget id="calculator">8 + 6 + 7</gadget>
<output>21</output>

<result>21</result>