Description

On the plane coordinate system, there are $N$ rectangles parallel to the axis. They may have a part of the cover of each other. Now do it just with area.

Format

Input

There is a number $N(1\leq N\leq 2000)$, means the number of rectangles.
The next $N$ lines, each line contains four integers, means the coordinates of the lower left corner and the upper right corner of this rectangle.(range of integer: $[-10^{8}, 10^{8}]$)

Output

Only one line, one integer represent the graphic area.

Samples

3
1 1 4 3
2 -1 3 2
4 0 5 2

10