INVENTORIES - Amount of inventories recognized as an expense and included in profit or loss (Details) - CNY (¥) ¥ in Thousands	12 Months Ended
Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
INVENTORIES
Carrying amount of inventories sold	¥ 211,292	¥ 303,021	¥ 443,382
Write down of inventories (reversal of inventory provision)	(2,301)	(56,766)	55,973
Cost of sales	¥ 208,991	¥ 246,255	¥ 499,355

The amount of costs relating to expenses directly or indirectly attributed to the goods or services sold, which may include, but are not limited to, costs previously included in the measurement of inventory that has now been sold, unallocated production overheads and abnormal amounts of production costs of inventories.

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