It is known that the perimeter of an isosceles trapezoid is 19 cm, the upper base is 3 cm, and the lower base is 5 cm shorter. What is the length of a waist of this trapezoid in cm?
已知等腰梯形的周长是19cm，上底是3cm，比下底短5cm，这个梯形的一个腰长是多少cm？
<gadget id="calculator">19 - 3 - 3 - 5</gadget>
<output>8</output>

<gadget id="calculator">8 / 2</gadget>
<output>4</output>

<result>4</result>