input: Write a python function to count down from a given number to 1 and print "Blast off!" at the end.
  assert countdown(3) == "3\n2\n1\nBlast off!"
  assert countdown(5) == "5\n4\n3\n2\n1\nBlast off!"
  assert countdown(1) == "1\nBlast off!"
output: def countdown(n):
      for i in range(n, 0, -1):
          print(i)
      print("Blast off!")