List all paper ID and its class type with more than 20 cited words.
class type refers to class_label; count(word_cited_id)>20
SELECT DISTINCT T1.paper_id, T1.class_label FROM paper AS T1 INNER JOIN content AS T2 ON T1.paper_id = T2.paper_id GROUP BY T2.paper_id, T1.class_label HAVING COUNT(T2.word_cited_id) > 20