Grass Planting Area

Farmer John bought a new machine that can plant grass in any axis‐aligned rectangular region of his farm. Unfortunately, the machine malfunctioned and planted grass in N rectangular regions, some of which may overlap. Given these rectangles, compute the total area covered by grass, counting overlapping areas only once.

In the Cartesian coordinate system (with the positive X axis pointing right and the positive Y axis pointing up), each rectangle is defined by its top‐left corner (x₁, y₁) and bottom‐right corner (x₂, y₂). Formally, if the i^th rectangle is specified by (x₁, y₁) and (x₂, y₂), its area is given by:

$$\text{Area}_i = \max(0, x_2-x_1) \times \max(0, y_1-y_2) $$

Note that if some portions of the rectangles overlap, the overlapping area should only be counted once in the final answer.

inputFormat

The first line contains an integer N (1 ≤ N ≤ 1000), the number of rectangles.

Each of the next N lines contains four integers: x1 y1 x2 y2, where (x1, y1) is the top‐left coordinate and (x2, y2) is the bottom‐right coordinate of a rectangle.

It is guaranteed that x1 < x2 and y1 > y2.

outputFormat

Output a single integer representing the total area covered by the N rectangles.

sample

1
0 10 10 0

100