Square Pool

Ron wants to build a square pool in his square N-by-N yard, but his yard contains T trees. Your job is to determine the side length of the largest square pool he can build.

The first line of input will be an integer N with N ≥ 2. The second line will be the positive integer T where T < N2. The remaining input will be T lines, each representing the location of a single tree. The location is given by two positive integers, R and then C, separated by a single space. Each tree is located at row R and column C where rows are numbered from top to bottom from 1 to N and columns are numbered from left to right from 1 to N. No two trees are at the same location.

Output one line containing M which is the largest positive integer such that some M-by-M square contained entirely in Ron’s yard does not contain any of the T trees.