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Fair value (Details) - USD ($)
Sep. 30, 2021
Dec. 31, 2020
Recurring    
Net liability arising from put/call options $ 582,000 $ 690,000
Fair Value, Inputs, Level 3 [Member]    
Recurring    
Net liability arising from put/call options 582,000 $ 690,000
Certificate of deposit [Member]    
Recurring    
Fair value, Assets 2,500,000  
Certificate of deposit [Member] | Fair Value, Inputs, Level 2 [Member]    
Recurring    
Fair value, Assets $ 2,500,000