def _pickup_dt(sys1, sys2):
    """
    Determine the sampling time of the new system.

    :param sys1: the first system
    :type sys1: TransferFunction
    :param sys2: the second system
    :type sys2: TransferFunction
    :return: sampling time
    :rtype: int | float
    """
    if sys1.dt is None and sys2.dt is not None:
        return sys2.dt
    elif sys1.dt is not None and sys2.dt is None or sys1.dt == sys2.dt:
        return sys1.dt
    else:
        msg = f'Expected the same sampling time. got sys1:{sys1.dt} sys2:{sys2.dt}'
        raise ValueError(msg)