Series (2 points)
=================

Given an integer, :math:`a`, we define the following sequence:

.. math::
   & t_1  = a \\
   & t_i  = t_{i-1}/i,  \quad  i\geq 2 \\

For example, the first 4 items of this sequence are: :math:`a, a/2,
a/6, a/24`

Write function :py:func:`series` that takes two integers, ``a`` and
``z`` (:class:`int`) and returns the number of terms of the sequence
(:class:`int`) that we need to add to get a value greater than or
equal to ``z``, i. e., the smallest index :math:`i` such that
:math:`\sum_{n=1}^{i}t_n \geq z`.

Save this function in file ``series.py``.

Examples:

.. literalinclude:: test-series.txt
      :language: python
      :lines: 3-10

.. note::

   More tests are provided in file :download:`test-series.txt`.