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SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Concentration risks (Details)
3 Months Ended 6 Months Ended
Jun. 30, 2019
USD ($)
customer
Jun. 30, 2018
USD ($)
customer
Jun. 30, 2019
USD ($)
customer
Jun. 30, 2018
USD ($)
customer
Concentration risks        
Sales, net $ 9,595,890 $ 7,474,993 $ 17,528,364 $ 15,147,794
Supplier Concentration Risk        
Concentration risks        
Concentration risk (as a percent) 84.10% 90.80% 80.50% 87.80%
Sales | Customer Concentration Risk        
Concentration risks        
Number of significant customers | customer 3 2 3 2
Sales, net $ 4,000,000 $ 3,100,000 $ 7,900,000 $ 6,300,000
Concentration risk (as a percent) 42.70% 41.60% 45.40% 41.80%