Unija

문제

You are given N rectangles, which are centered in the center of the Cartesian coordinate system and their sides are parallel to the coordinate axes. Each rectangle is uniquely identified with its width (along the x-axis) and height (along the y-axis). The lower image depicts the first sample test.

Mirko has coloured each rectangle in a certain color and now wants to know the area of the coloured part of the paper. In other words, he wants to know the number of unit squares tha t belong to at least one rectangle.

입력

The first line of input contains the integer N (1 ≤ N ≤ 1 000 000), the number of rectangles.

Each of the following N lines contains even integers X and Y (2 ≤ X, Y ≤ 10⁷), dimensions (width and height, respectively) of the corresponding rectangles.

출력

The first and only line of output must contain the required area.

3
8 2
4 4
2 6

28

5
2 10
4 4
2 2
8 8
6 6

68