input: Write a function to create a Python program that takes in a positive integer, then uses a while loop to output the numbers counting down to 1. The function should take one parameter, the positive integer, and print each number starting from the integer down to 1, followed by the word "Done!".
  
  assert count_down(6) == [6, 5, 4, 3, 2, 1]
  assert count_down(3) == [3, 2, 1]
  assert count_down(1) == [1]
  assert count_down(0) == "Invalid input"
  assert count_down(-5) == "Invalid input"
output: def count_down(n):
      if n < 0:
          return "Invalid input"
      while n > 0:
          print(n)
          n -= 1
      print("Done!")