Venue Rental (Small)

Sanghyun is about to leave for military service and wants to throw a farewell party. He rents a venue for it, and the rental company is unusual because only former programmers work there.

Once Sanghyun picks the place he wants, the company builds $N$ rectangles that cover that place completely. It then tells him the number of rectangles $N$ and, for each rectangle, the coordinates of its bottom left corner and its top right corner. The $N$ rectangles may overlap partially or completely. Every rectangle has sides parallel to the coordinate axes.

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

The first line contains the number of rectangles $N$ ($2 \le N \le 100$).

Each of the next $N$ lines contains four integers $x_1$, $y_1$, $x_2$, $y_2$ ($0 \le x_1 < x_2 \le 500$, $0 \le y_1 < y_2 \le 500$) separated by spaces. The bottom left corner of that rectangle is $(x_1, y_1)$ and the top right corner is $(x_2, y_2)$.

Print the area of the venue Sanghyun rented on the first line. The area is always an integer, so print it without a decimal point.