def is_prime(n: int) -> bool:
    """
    Return True if n is prime, False otherwise.
    """
    if n < 2:
        return False
    elif n < 4:
        return True
    elif n % 2 == 0 or n % 3 == 0:
        return False
    else:
        for i in range(5, int(n**0.5)+1, 6):
            if n % i == 0 or n % (i+2) == 0:
                return False
        return True