A1356 | [COCI-2016_2017-contest5]#6 Unija

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 that belong to at least one rectangle.

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.

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