Bishop

You are given an N*N chessboard. A bishop can move any distance along each of the four diagonal directions from its current square, and it can attack another bishop on the same diagonal.

Place as many bishops as possible on the board so that no two bishops attack each other. Given the board size N, write a program that prints the maximum number of bishops that can be placed.

The board size N is given. N is a positive integer written in decimal notation, and it has at most 70 digits.

Print the maximum number of bishops that can be placed so that no two attack each other.