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Income taxes - Reconciliation of the differences between PRC statutory income tax rate and the Group's effective income tax rates (Details)
12 Months Ended
Dec. 31, 2022
Dec. 31, 2021
Dec. 31, 2020
Income taxes      
Computed expected income tax benefit (25.00%) (25.00%) (25.00%)
Effect of preferential tax rate (18.00%) 10.00% 11.00%
Effect of different tax jurisdiction 2.00% (1.00%) (3.00%)
Non-deductible expenses 40.00% 5.00% 4.00%
Research and development expenses additional deduction (5.00%) (6.00%) (8.00%)
Change in valuation allowance 7.00% 17.00% 21.00%
Actual income tax expense 1.00%