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Note 9 - Preferred Stock and Common Stock Warrants (Tables)
6 Months Ended
Jun. 30, 2020
Notes Tables  
Fair Value Measurement Inputs and Valuation Techniques [Table Text Block]
  

June 30,

  

December 31,

 
  

2020

  

2019

 

Stock price

 $2.16  $6.14 

Exercise price

 $0.01  $0.01 

Risk-free rate

  0.2%  1.6%

Volatility

  61.3%  27.6%

Term (years)

  1.3   1.8