input: 
  def is_prime(num: int) -> str:
      """ Determine if the provided number is prime.
      >>> is_prime(17)
      '17 is a prime number.'
      >>> is_prime(18)
      '18 is not a prime number.'
      """
      is_prime = True
      for i in range(2, num):
          if num % i == 0:
              is_prime = False
              break
      if is_prime:
          return f'{num} is a prime number.'
      else:
          return f'{num} is not a prime number.'
  
output:     if num <= 1:
          return f'{num} is not a prime number.'