input: 
  def is_prime(num: int) -> str:
      """ Check if the given number is a prime number.
      >>> is_prime(17)
      '17 is a prime number.'
      >>> is_prime(20)
      '20 is not a prime number.'
      """
      is_prime = True
      for i in range(2, num):
          if num % i == 0:
              is_prime = False
              break
      return f'{num} is a prime number.' if is_prime else f'{num} is not a prime number.'
  
output:     is_prime = True
      for i in range(2, int(num**0.5) + 1):
          if num % i == 0:
              is_prime = False
              break
      return f'{num} is a prime number.' if is_prime else f'{num} is not a prime number.'