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Accounts receivable - Movement of allowance for doubtful accounts (Details) - USD ($)
12 Months Ended
Dec. 31, 2021
Dec. 31, 2020
Accounts receivable    
Balance at beginning of period $ 3,699,890 $ 5,731,281
Change of allowance for doubtful accounts (53,436) (895,043)
Write off (15,631) (1,523,489)
Translation adjustments 86,885 387,141
Balance at end of period $ 3,717,708 $ 3,699,890