Income taxes - Provision for income taxes (Details) - USD ($)	12 Months Ended
Dec. 31, 2019	Dec. 31, 2018
Current:
Federal	$ 143,567	$ 100,806
State	171,093	106,922
Total	314,660	207,728
Deferred Federal, State and Local, Tax Expense (Benefit) [Abstract]
Federal	(203,031)	(302,440)
State	(45,008)	(62,579)
Total	(248,039)	(365,019)
Income Tax Expense (Benefit)	$ 66,621	$ (157,291)

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