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Note 4 - Stock-based Compensation - Summary of Stock Option Activity (Details) - USD ($)
6 Months Ended
Jun. 30, 2017
Jun. 30, 2016
Outstanding, number of shares (in shares) 68,400
Outstanding, weighted average remaining contractual term (Year) 4 years 361 days
Outstanding, aggregate intrinsic value $ 83,933
Exercisable, weighted average exercise price (in dollars per share) $ 5.17
Exercisable, number of shares (in shares) 68,400
Exercisable, weighted average remaining contractual term (Year) 4 years 361 days
Exercisable, aggregate intrinsic value $ 83,933
Outstanding, weighted average exercise price (in dollars per share) $ 5.14 $ 5.17
Outstanding, number of shares (in shares) 56,400 68,400
Granted, number of shares (in shares)
Cancelled, weighted average exercise price (in dollars per share) $ 5.14
Cancelled, number of shares (in shares) (12,000)
Exercised, weighted average exercise price (in dollars per share) $ 5.14  
Exercised, number of shares (in shares) (44,400)
Exercised, weighted average remaining contractual term (Year) 3 years 361 days  
Exercised, aggregate intrinsic value $ 155,606  
Outstanding, weighted average exercise price (in dollars per share) $ 5.17