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Accounts receivable (Details) - USD ($)
Dec. 31, 2021
Dec. 31, 2020
Accounts receivable    
Accounts receivable $ 48,680,634 $ 38,110,487
Allowance for doubtful accounts (3,717,708) (3,699,890)
Accounts receivable, net $ 44,962,926 $ 34,410,597