There is an infinite square grid. Some vertices of the grid are black and other vertices are white.
A vertex V is called inner if it is both vertical-inner and horizontal-inner. A vertex V is called horizontal-inner if there are two such black vertices in the same row that V is located between them. A vertex V is called vertical-inner if there are two such black vertices in the same column that V is located between them.
On each step all white inner vertices became black while the other vertices preserve their colors. The process stops when all the inner vertices are black.
Write a program that calculates a number of black vertices after the process stops.
The input test file will contain multiple test cases. The first line of the test case contains one integer
number n (0 <= n <= 100 000) — number of black vertices at the beginning.
The following n lines contain two integer numbers each — the coordinates of different black vertices. The
coordinates do not exceed 109 by their absolute values.
For each test case, output the number of black vertices when the process stops. If the process does not
stop, output -1.
4 0 2 2 0 -2 0 0 -2
5
Migrated from old NTUJ.
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