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Stock-based compensation - Assumptions to determine grant-date fair value of stock options (Details) - Stock options
12 Months Ended
Mar. 31, 2021
Mar. 31, 2020
Stock-based Compensation    
Risk-free interest rate 0.93% 2.12%
Expected term (in years) 6 years 3 months 18 days 6 years
Expected volatility 75.40% 71.40%
Expected dividend yield 0.00% 0.00%