SUMMARY OF PRINCIPAL ACCOUNTING POLICIES - Selling and marketing expenses and Others (Details) - CNY (¥) ¥ in Thousands	12 Months Ended
Dec. 31, 2022	Dec. 31, 2021	Dec. 31, 2020
Selling and marketing expenses
Advertising expenses	¥ 35,647	¥ 30,135	¥ 37,577
Government grants
Government grants	13,190	2,947	5,036
Income taxes
Unrecognized tax benefits	¥ 0	¥ 0	¥ 0
Value added taxes
Minimum percentage value added tax	0.00%
Maximum percentage value added tax	13.00%

Amount of unrecognized tax benefits.

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 740
-SubTopic 10
-Section 45
-Paragraph 10B
-URI https://asc.fasb.org/extlink&oid=123427490&loc=SL37586934-109318

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 740
-SubTopic 10
-Section 50
-Paragraph 15A
-Subparagraph (a)
-URI https://asc.fasb.org/extlink&oid=121826272&loc=SL6600010-109319

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Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant