input: Write a function to return the maximum number in a list of integers. Return the maximum number if the list is not empty, otherwise return -1.
  assert find_max([1, 15, 20, 6])==20
  assert find_max([5, -3, 25, 9])==25
  assert find_max([-5, -10, -1])==-1
  assert find_max([10])==10
  assert find_max([])==-1
output: def find_max(lst):
      if not lst:
          return -1
      return max(lst)