input: Write a function to find and return the maximum number from a given list of integers. If the list is empty, return a string 'List is empty'.
  assert max_num([5, 4, 2, 7, 12])=='12'
  assert max_num([4, 3, 7, 9])=='9'
  assert max_num([1])=='1'
  assert max_num([])=='List is empty'
output: def max_num(lst):
      if not lst:
          return 'List is empty'
      return str(max(lst))