Summary of significant accounting policies - Employee benefit (Details) - USD ($)	12 Months Ended
Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2017
Summary of significant accounting policies
Total expense for the plans	$ 95,331	$ 54,804	$ 51,801

Amount of discretionary contributions made by an employer to a defined contribution plan.

No definition available.

Name:	us-gaap_DefinedContributionPlanEmployerDiscretionaryContributionAmount
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration