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Intangible Assets, net (Tables)
9 Months Ended
Sep. 30, 2021
Goodwill and Intangible Assets Disclosure [Abstract]  
Schedule of intangible assets
   December 31,   September 30, 
   2020   2021 
Computer software at cost  $32,686   $44,634 
Accumulated amortization   (20,879)   (23,216)
   $11,807   $21,418