#include <bits/stdc++.h>
#include <x86intrin.h>
using namespace std;
using namespace numbers;



int main(){
	cin.tie(0)->sync_with_stdio(0);
	cin.exceptions(ios::badbit | ios::failbit);
	int nr, nc;
	cin >> nr >> nc;
	vector<string> a(nr);
	copy_n(istream_iterator<string>(cin), nr, a.begin());
	vector<vector<int>> cnt(nr + 1, vector<int>(nc + 1));
	for(auto x = 0; x < nr; ++ x){
		for(auto y = 0; y < nc; ++ y){
			cnt[x + 1][y + 1] = (a[x][y] == '#') + cnt[x][y + 1] + cnt[x + 1][y] - cnt[x][y];
		}
	}
	auto count = [&](int xl, int xr, int yl, int yr)->int{
		return cnt[xl][yl] - cnt[xl][yr] - cnt[xr][yl] + cnt[xr][yr];
	};
	vector<vector<int>> push(nr + 1, vector<int>(nc + 1));
	cout << *ranges::partition_point(ranges::iota_view(1, min(nr, nc) + 1), [&](int k){
		ranges::fill(push | ranges::views::join, 0);
		for(auto x = 0; x + k <= nr; ++ x){
			for(auto y = 0; y + k <= nc; ++ y){
				if(k * k == count(x, x + k, y, y + k)){
					push[x][y] += 1;
					push[x][y + k] += -1;
					push[x + k][y] += -1;
					push[x + k][y + k] += 1;
				}
			}
		}
		for(auto x = 0; x < nr; ++ x){
			for(auto y = 0; y < nc; ++ y){
				push[x][y + 1] += push[x][y];
			}
		}
		for(auto x = 0; x < nr; ++ x){
			for(auto y = 0; y < nc; ++ y){
				push[x + 1][y] += push[x][y];
			}
		}
		for(auto x = 0; x < nr; ++ x){
			for(auto y = 0; y < nc; ++ y){
				if(!!push[x][y] != (a[x][y] == '#')){
					return false;
				}
			}
		}
		return true;
	}) - 1 << "\n";
	return 0;
}

/*

*/

////////////////////////////////////////////////////////////////////////////////////////
//                                                                                    //
//                                   Coded by Aeren                                   //
//                                                                                    //
////////////////////////////////////////////////////////////////////////////////////////