What is the length of the shortest open-closed tube needed to produce this frequency?

The whole question is;
The lowest frequency in the audible range is 20Hz.
What is the length of the shortest open-closed tube needed to produce this frequency?
The fundamental resonant frequency of the tube opened at one end is given by this formula:

Where v_s is the speed of sound in the air, and L is the length of the tube.
The speed of sound at 20 degrees Celsius is v_s=343.7m/s. 
So the shortest length is:

50 meters per 5 seconds