List the report number of crimes against property happened in Riverdale.
crime against property refers to crime_against = 'Property'; Riverdale refers to community_area_name = 'Riverdale'
SELECT SUM(CASE WHEN T1.crime_against = 'Property' THEN 1 ELSE 0 END) FROM FBI_Code AS T1 INNER JOIN Crime AS T2 ON T2.fbi_code_no = T1.fbi_code_no INNER JOIN Community_Area AS T3 ON T3.community_area_no = T2.community_area_no WHERE T3.community_area_name = 'Riverdale'