A pair of overlap aerial photographs was taken with a camera from an altitude of 3000 m above datum. The focal length of camera was 120 mm and the mean distance between two principal points, both of which lie on the datum, was 71.20 mm. In the common overlap area, a tall chimney 200 m high with its base in the datum surface was observed. The difference of parallax for the top and bottom of the chimney is ________mm.