Among the parts that are returned, how many of them are provided by a supplier in debt?
returned refers to l_returnflag = 'R'; in debt refers to s_acctbal < 0
SELECT COUNT(T1.l_partkey) FROM lineitem AS T1 INNER JOIN supplier AS T2 ON T1.l_suppkey = T2.s_suppkey WHERE T1.l_returnflag = 'R' AND T2.s_acctbal < 0