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SIGNIFICANT ACCOUNTING POLICIES - Advertising costs (Details) - CNY (¥)
¥ in Thousands
12 Months Ended
Dec. 31, 2020
Dec. 31, 2019
Dec. 31, 2018
Significant Accounting Policies      
Advertising Expense ¥ 7,474 ¥ 10,664 ¥ 8,450