def pass_dummy_scans(algo_dummy_scans, dummy_scans=None):
    """
    Graft manually provided number of dummy scans, if necessary.

    Parameters
    ----------
    algo_dummy_scans : int
        number of volumes to skip determined by an algorithm
    dummy_scans : int or None
        number of volumes to skip determined by the user

    Returns
    -------
    skip_vols_num : int
        number of volumes to skip

    """
    if dummy_scans is None:
        return algo_dummy_scans
    return dummy_scans