input: Write a function to find the maximum number from a list of non-negative integers. If the list is empty, return -1.
  assert find_max([1, 3, 12, 5, 8, 10])==12
  assert find_max([0, 0, 0, 0])==0
  assert find_max([])==-1
  assert find_max([4, 6, 3, 1, 9])==9
output: def find_max(lst):
      if not lst:
          return -1
      return max(lst)