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

//* ATCODER
#include<atcoder/all>
using namespace atcoder;
typedef modint998244353 mint;
//*/

/* BOOST MULTIPRECISION
#include<boost/multiprecision/cpp_int.hpp>
using namespace boost::multiprecision;
//*/

typedef long long ll;

#define rep(i, s, n) for (int i = (int)(s); i < (int)(n); i++)
#define rrep(i, s, n) for (int i = (int)(n)-1; i >= (int)(s); i--)

template <typename T> bool chmin(T &a, const T &b) {
	if (a <= b) return false;
	a = b;
	return true;
}

template <typename T> bool chmax(T &a, const T &b) {
	if (a >= b) return false;
	a = b;
	return true;
}

template <typename T> T max(vector<T> &a){
	assert(!a.empty());
	T ret = a[0];
	for (int i=0; i<(int)a.size(); i++) chmax(ret, a[i]);
	return ret;
}

template <typename T> T min(vector<T> &a){
	assert(!a.empty());
	T ret = a[0];
	for (int i=0; i<(int)a.size(); i++) chmin(ret, a[i]);
	return ret;
}

template <typename T> T sum(vector<T> &a){
	T ret = 0;
	for (int i=0; i<(int)a.size(); i++) ret += a[i];
	return ret;
}

int main(){
	int h, w; cin >> h >> w;
	vector s(h, vector<char>(w));
	rep(i,0,h){
		rep(j,0,w){
			cin >> s[i][j];
		}
	}
	vector rui(h+1, vector<int>(w+1));
	rep(i,0,h){
		rep(j,0,w){
			if (s[i][j] == '#'){
				rui[i+1][j+1]++;
			}
		}
	}
	rep(i,0,h+1){
		rep(j,0,w){
			rui[i][j+1] += rui[i][j];
		}
	}
	rep(i,0,h){
		rep(j,0,w+1){
			rui[i+1][j] += rui[i][j];
		}
	}
	int ub = min(h, w) + 1;
	int lb = 1;
	while (ub - lb > 1){
		int t = (ub + lb) / 2;
		vector imos(h+1, vector<int>(w+1));
		rep(i,0,h-t+1){
			rep(j,0,w-t+1){
				if (rui[i+t][j+t] - rui[i][j+t] - rui[i+t][j] + rui[i][j] == t * t){
					imos[i][j]++;
					imos[i+t][j]--;
					imos[i][j+t]--;
					imos[i+t][j+t]++;
				}
			}
		}
		rep(i,0,h+1){
			rep(j,0,w){
				imos[i][j+1] += imos[i][j];
			}
		}
		rep(i,0,h){
			rep(j,0,w+1){
				imos[i+1][j] += imos[i][j];
			}
		}
		bool mode = true;
		rep(i,0,h){
			rep(j,0,w){
				if (imos[i][j] == 0 && s[i][j] == '#'){
					mode = false;
				}
			}
		}
		if (mode){
			lb = t;
		}else{
			ub = t;
		}
	}
	cout << lb << '\n';
}