A smart student finds an old water well in the forest. She wants to know its depth, but there is fog inside the well and she cannot see the water surface. However, she has a cell phone that can act as portable tone generator. It can produce audible tones with frequencies starting from 100 Hz. By sending the sound down the well and listening to the well's "response", she finds that the air column in the well resonates at frequencies of 117 Hz and 143 Hz, and there are no resonant frequencies between these two. Find how deep is the well, knowing that the temperature of the air inside the well is 20 degrees Celsius.