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Summary of significant accounting policies - Currency exchange rates (Details) - $ / ¥
Jun. 30, 2022
Dec. 31, 2021
Jun. 30, 2021
Foreign Currency Translation      
Currency exchange rate (US$:RMB) 0.1493 0.1569 0.1549
Currency exchange rate (US$:RMB) - Average 0.1543 0.1550 0.1546