input: Write a function that takes a positive integer as input and uses a while loop to output the numbers counting down to 1. If the input is not a positive integer, the function should print an error message and exit. The function should return nothing.
  
  assert countdown(6)
  assert countdown(10)
  assert countdown(1)
  assert countdown(-1)
  assert countdown(0)
output: def countdown(n):
      if n <= 0:
          print("Error: Input must be a positive integer")
          return
      while n > 0:
          print(n)
          n -= 1