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Summary of significant accounting policies - Foreign currency translation and transaction (Details)
Jun. 30, 2019
USD ($)
Jun. 30, 2018
USD ($)
Jun. 30, 2017
Summary of significant accounting policies      
Translation adjustments included in accumulated other comprehensive loss $ (308,571) $ (41,025)  
Historical rate 6.87 6.62  
Average translation rate 6.83 6.51 6.81