Database: academic
Question: return me the total citations of the papers in " University of Michigan " .
SQL:
SELECT SUM ( t4.citation_num ) FROM organization AS t2 JOIN author AS t1 ON t2.oid  =  t1.oid JOIN writes AS t3 ON t3.aid  =  t1.aid JOIN publication AS t4 ON t3.pid  =  t4.pid WHERE t2.name  =  "University of Michigan";