def triangular(n):
    """
    The triangular numbers are the numbers 1, 3, 6, 10, 15, 21, ...
    They are calculated as follows.
    1 = 1
    1 + 2 = 3
    1 + 2 + 3 = 6
    1 + 2 + 3 + 4 = 10
    1 + 2 + 3 + 4 + 5 = 15
    Returns nth triangular number.
    """
    return sum([i for i in range(n+1)])