def format_agg_rois(rois):
    """
    Helper function to format MultiImageMaths command.

    Parameters
    ----------
    rois : `list` of `str`s
        List of files

    Returns
    -------
    first_image
    op_files
    op_string

    """
    return rois[0], rois[1:], ("-add %s " * (len(rois) - 1)).strip()