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Note 3 - Stock-based Compensation; Changes in Equity (Tables)
 6 Months Ended
Jun. 30, 2018
Notes Tables  
Share-based Compensation, Activity [Table Text Block]
   
Shares
under
Option
    
Weighted
Average
Exercise
Price
  
Balance at December 31, 2017
    
476,144
     $
3.97
  
Granted
    
-
      
-
  
Cancelled/Expired
    
-
      
-
  
Exercised/Issued
    
(5,000
 )    
-
  
Outstanding at June 30, 2018
    
471,144
     $
4.01
  
                 
Exercisable at June 30, 2018
    
157,639
     $
4.84