It is also a two-segment monotone decreasing interval. If it is "union", it means that the two segments are regarded as a whole, and the whole is still monotonically decreasing, that is, the maximum value above [1, 4] is less than the minimum value above [-4, 2]. If it is "sum", there is no such requirement, as long as it decreases monotonically on [1, 4] and [-4, 2] respectively.