Database: academic_minus_1_2
Question: return me the number of papers in VLDB conference in " University of Michigan " .
SQL: select count ( distinct t3.title ) from organization_author as t1 join writes as t2 on t1.aid = t2.aid join publication as t3 on t2.pid = t3.pid join conference as t4 on t3.cid = t4.cid where t4.name = 'vldb' and t1.name_organization = 'university of michigan' ;