Among the cars on a train that runs in the east direction, how many of them have a flat roof and a circle load shape?
flat roof refers to roof = 'flat'; load_shape = 'circle'
SELECT SUM(CASE WHEN T1.load_shape = 'circle' THEN 1 ELSE 0 END)as count FROM cars AS T1 INNER JOIN trains AS T2 ON T1.train_id = T2.id WHERE T2.direction = 'east' AND T1.roof = 'flat' 