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Summary of significant accounting policies - Currency exchange rates (Details)
Dec. 31, 2021
$ / ¥
Dec. 31, 2020
$ / ¥
Dec. 31, 2019
$ / ¥
Foreign Currency Translation      
Foreign Currency Exchange Rate, Translation 0.1569 0.1533 0.1436
Foreign Currency Average Exchange Rate Translation 0.1550 0.1448 0.1448