For a colored ribbon, half of the full length was cut off for the first time, and the remaining half was cut off for the second time, which was 4 meters short, leaving 9 meters left. How long is this ribbon in meters?
一根彩带，第一次剪去了全长的一半，第二次又剪了余下的一半少4米，还剩9米．这根彩带全长多少米？
<gadget id="calculator">9 - 4</gadget>
<output>5</output>

<gadget id="calculator">5 * 2</gadget>
<output>10</output>

<gadget id="calculator">10 * 2</gadget>
<output>20</output>

<result>20</result>