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ACCOUNTS RECEIVABLE, NET - Allowance for doubtful accounts (Details) - CNY (¥)
¥ in Thousands
6 Months Ended 12 Months Ended
Jun. 30, 2021
Dec. 31, 2020
Allowance for doubtful accounts:    
Balance at beginning of year/Period ¥ (6,247) ¥ (2,993)
Addition (2,002) (7,857)
Written off 1,036 4,603
Balance at end of year/period ¥ (7,213) ¥ (6,247)