def pentagonal(nth):
    """
    Providing n'th pentagonal number.

    :param nth: index for n'th pentagonal
    :returns: n'th pentagonal number
    see http://en.wikipedia.org/wiki/Pentagonal_number

    >>> pentagonal(3)
    12
    >>> pentagonal(4)
    22
    """
    return (nth * (3 * nth - 1)) // 2