Triangular numbers are made by adding natural numbers in order: 1, 1 + 2, 1 + 2 + 3, and so on. The same idea can be written with the formula t(k) = k * (k + 1) / 2. Write a function is_triangular that checks whether n can be produced this way. Return True if n is triangular, otherwise return False.

def is_triangular(n: int) -> bool: