Colored Paper

There is a white square sheet of paper whose width and height are both 100. Black square papers, each with width and height 10, are placed on it. Every black paper is placed parallel to the sides of the white sheet.

Given one or more black papers, write a program that finds the area covered by black paper.

If multiple papers overlap, the overlapped area must be counted only once.

The first line contains the number of black papers.

From the second line, each line gives the position of one black paper. Each position is given as two natural numbers. The first number is the distance between the left side of the black paper and the left side of the white sheet, and the second number is the distance between the bottom side of the black paper and the bottom side of the white sheet.

The number of black papers is at most 100, and no paper extends outside the white sheet.

Print the area covered by black paper on the first line.