cassidoo newsletter paint problem

paint.py

from math import ceil

def surface_area(l, w, h):
  return 2 * (w*l+h*l+h*w)

# only four walls and roof, we don't paint floor
def room_surface(l, w, h):
  return surface_area(l, w, h) - (2 * l * w)

def number_of_cans(room, can_coverage):
  return ceil(room_surface(
    room['length'],
    room['width'],
    room['height']
  ) / can_coverage)

room = { "length": 12, "width": 10, "height": 9 }
can_coverage = 200

print(number_of_cans(room, can_coverage))