It is known that the perimeter of an isosceles trapezoid is 38 cm, the length of the upper base is 4 cm, and the length of the lower base is twice the length of the upper base. What is the waist length of this isosceles trapezoid in centimeters?
已知一个等腰梯形的周长是38cm，上底长4cm，下底长是上底长的2倍，这个等腰梯形的腰长是多少厘米？
<gadget id="calculator">4 * 2</gadget>
<output>8</output>

<gadget id="calculator">38 - 4 - 8</gadget>
<output>26</output>

<gadget id="calculator">26 / 2</gadget>
<output>13</output>

<result>13</result>