A tractor weighing 20 kN has a wheelbase of 2.0 m. In static condition on level ground, 35% of the total weight rests on the front axle. When pulling a horizontal drawbar load at a drawbar height of 0.40 m above ground, the maximum horizontal drawbar pull in kN before the front wheels lose contact with the ground ($R_f = 0$) is ________ (in integer).