B'You are given a board with n rows and n columns, numbered from 1 to n . The intersection of the a -th row and b -th column is denoted by (a, b) . A half-queen attacks cells in the same row, same column, and on one diagonal. More formally, a half-queen on (a, b) attacks the cell (c, d) if a=c or b=d or a-b=c-d . Construct an optimal solution. The first line contains a single integer n ( 1 <= n <= 10^5 ) -- the size of the board. In the first line print a single integer k -- the minimum number of half-queens. In each of the next k lines print two integers a_i , b_i ( 1 <= a_i, b_i <= n ) -- the position of the i -th half-queen. If there are multiple solutions, print any. Example 1 : one half-queen is enough. Note: a half-queen on (1, 1) attacks (1, 1) . Example 2 : one half-queen is enough too. (1, 2) or (2, 1) would be wrong solutions, because a half-queen on (1, 2) does not attack the cell (2, 1) and vice versa. (2, 2) is also a valid solution. Example 3 : it is impossible to cover the board with one half queen. There are multiple solutions for 2 half-queens; you can print any of them. '...