Database: academic
Question: return me the number of papers in VLDB conference in " University of Michigan " .
SQL: select count ( distinct t5.title ) from organization as t3 join author as t1 on t3.oid  =  t1.oid join writes as t4 on t4.aid  =  t1.aid join publication as t5 on t4.pid  =  t5.pid join conference as t2 on t5.cid  =  t2.cid where t2.name  =  "vldb" and t3.name  =  "university of michigan";