input: 
  def is_prime(num: int) -> str:
      """ Checks if given number is prime or not and returns a string stating the result.
      >>> is_prime(17)
      '17 is a prime number.'
      >>> is_prime(20)
      '20 is not a prime number.'
      """
  
  
output:     if num <= 1:
          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."