Venue Rental (Large)

Sanghyun is planning a farewell party before he leaves for his military service, so he wants to rent a venue for it.

Everyone who works at the rental company is a former programmer, which makes the rental procedure unusual. Once Sanghyun picks the place he wants, the company builds $N$ rectangles that cover that place exactly. It then tells him the number of rectangles $N$ and, for each rectangle, the coordinates of its lower left corner and its upper right corner. The $N$ rectangles may overlap in part or in full. Every side of every rectangle is parallel to a coordinate axis.

Write a program that computes the area of the venue Sanghyun rented, which is the area of the region covered by the $N$ rectangles.

The first line contains the number of rectangles $N$. ($2 \le N \le 3000$)

Each of the next $N$ lines contains four integers $x_1$, $y_1$, $x_2$, $y_2$ in this order. ($-50000 \le x_1 < x_2 \le 50000$, $-50000 \le y_1 < y_2 \le 50000$) The lower left corner of that rectangle is $(x_1, y_1)$ and its upper right corner is $(x_2, y_2)$.

Print the area of the venue Sanghyun rented on the first line. Count an overlapping part only once. The area reaches $10^{10}$, so it does not fit in a 32-bit integer.