## P1050 Union of Rectangles

**Description**

A rectangle in a plane having its sides parallel to the coordinate axis, is determined by the coordinates of then leftdown and righttop vertex.

Write a program for computing the area of the union of a given sequence of rectangles A_1,A_2,...,A_N.

*Example:*

A_1 is given by [1;1],[4;3] A_2 is given by [0;1],[6;2] A_3 is given by [2;1],[3;3] A_4 is given by [7;7],[9;9] The area of the union is 13.

**Input**

第一行输入矩形的个数，以后每行输入左下角和右顶点的坐标，用空格分隔。

**Output**

输出所有矩形的并集的面积。

**Sample Input**

**Sample Output**