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Organization, Use of Estimates and Basis of Presentation - Summary of Potentially Antidilutive Securities (Details) - shares
3 Months Ended
Jul. 31, 2017
Jul. 31, 2016
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]    
Total common stock equivalents 4,506,090 4,264,788
Stock options    
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]    
Total common stock equivalents 2,501,806 2,154,948
Warrants    
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]    
Total common stock equivalents 2,004,284 2,109,840