What is the connection between number 20 and 21930?

The Jiankou section, one of the most treacherous parts of the Great Wall, has seen incredible dedication from Cheng Yongmao. As a technical advisor for the restoration, he's spent 20 years restoring 21,930 meters of the Wall in Beijing's Huairou District. "No one knows these bricks and stones better than I do," he says.


