diff --git a/build.py b/build.py
index 625a0a825278e9f779685aebfb94027355d21a9a..2f57d8c32746c0d6338c7fbe8aa1ce7e44bdbc22 100644
--- a/build.py
+++ b/build.py
@@ -29,7 +29,7 @@ if __name__ == '__main__':
             for k, v in d.iteritems():
                 lab = lab.replace(u'$%s$' % (k, ), v)
 
-            with open(dist_lp+'%s.md' % (ex_no, ), 'wb') as labout:
+            with open('%s%s.md' % (dist_lp, ex_no, ), 'wb') as labout:
                 labout.write(lab.encode('utf8'))
 
             os.system(('pandoc %s%s.md -s -o %s%s.pdf') % (dist_lp, ex_no, dist_lp, ex_no))