input: def is_prime(num: int) -> str:
      """Determine if the provided number is prime or not.
      >>> is_prime(17)
      '17 is a prime number.'
      >>> is_prime(18)
      '18 is not a prime number.'
      """
      if num < 2:
          return f'{num} is not a prime number.'
      for i in range(2, num):
          if num % i == 0:
              return f'{num} is not a prime number.'
      return f'{num} is a prime number.'
output:     if num < 2:
          return f'{num} is not a prime number.'
      for i in range(2, int(num**0.5) + 1):
          if num % i == 0:
              return f'{num} is not a prime number.'
      return f'{num} is a prime number.'