def _get_font_size(document, style):
    """Get font size defined for this style.

    It will try to get font size from it's parent style if it is not defined by original style. 

    :Args:
      - document (:class:`ooxml.doc.Document`): Document object
      - style (:class:`ooxml.doc.Style`): Style object

    :Returns:
      Returns font size as a number. -1 if it can not get font size.
    """

    font_size = style.get_font_size()

    if  font_size == -1:
        if style.based_on:
            based_on = document.styles.get_by_id(style.based_on)
            if based_on:
                return _get_font_size(document, based_on)

    return font_size