# We want make a package of goal kilos of chocolate. 
# We have small bars (1 kilo each) and big bars (5 kilos each). 
# Return the number of small bars to use, assuming we always use big bars before small bars. 
# Return -1 if it can't be done.

# make_chocolate(4, 1, 9) → 4
# make_chocolate(4, 1, 10) → -1
# make_chocolate(4, 1, 7) → 2

def make_chocolate(small, big, goal):
  big_needed = goal // 5
  
  if (big_needed <= big):
    small_needed = goal % 5
  else:
    small_needed = goal - (big * 5)
  
  if (small_needed <= small):
     return small_needed
  else:
    return -1