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SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Share-based compensation (Details) - USD ($)
12 Months Ended
Dec. 31, 2019
Dec. 31, 2018
Dec. 31, 2017
Dec. 31, 2016
Share-based compensation expense $ 0 $ 0 $ 672,223  
Cost of sales        
Share-based compensation expense     272,811  
Sales and marketing        
Share-based compensation expense     143,255  
Research and development        
Share-based compensation expense     45,174  
General and administrative        
Share-based compensation expense     $ 210,983  
Stock options        
Options amortization period       12 months
Amortized period for options granted     4 months