Write SQL query to solve given problem: What is the district number that Chris Van Hollen serving?. Keep the solution inside sql tag ```sql [SQL-Query] ```
SELECT T2.district FROM current AS T1 INNER JOIN `current-terms` AS T2 ON T1.bioguide_id = T2.bioguide WHERE T1.official_full_name = 'Chris Van Hollen' AND T2.district IS NOT NULL GROUP BY T2.district