def ackermann_naive(m: int, n: int) -> int:
    """Ackermann number.
    """
    if m == 0:
        return n + 1
    elif n == 0:
        return ackermann(m - 1, 1)
    else:
        return ackermann(m - 1, ackermann(m, n - 1))
Ackermann number.