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Pledged deposits (Tables)
3 Months Ended
Mar. 31, 2021
Pledged Deposits [Abstract]  
Schedule of pledged deposits
   December 31,   March 31, 
   2020   2021 
Pledged deposits with bank for:        
Bills payable  $8,791,499   $16,278,065 
Others*   198,249    45,238 
   $8,989,748   $16,323,303