Find the total area covered by two rectilinear rectangles in a 2D plane.
Each rectangle is defined by its bottom left corner and top right corner as shown in the figure.
Assume that the total area is never beyond the maximum possible value of int.
简单平面几何。根据容斥原理：S(M ∪ N) = S(M) + S(N) - S(M ∩ N)，最关键的是求出相交部分的面积。
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