#ifdef __GNUC__
#pragma GCC optimize ("O3")
#pragma GCC target ("avx")
#endif

#define _USE_MATH_DEFINES

#include <iostream>
#include <iomanip>
#include <stdio.h>

#include <sstream>
#include <algorithm>
#include <cmath>

#include <string>
#include <cstring>
#include <vector>
#include <valarray>
#include <array>

#include <queue>
#include <complex>
#include <set>
#include <map>
#include <stack>
#include <list>

#include <cassert>//assert();
#include <fstream>
/////////
#define REP(i, x, n) for(int i = x; i < n; i++)
#define rep(i,n) REP(i,0,n)

/////////
typedef long long LL;
typedef long double LD;
typedef unsigned long long ULL;
#define PII pair<int,int>
/////////
using namespace::std;

// 最大公約数
template<class T>
inline T gcd(T a, T b){return b == 0 ? a : gcd(b, a % b);}
// 最小公倍数
template<class T>
inline T lcm(T a, T b){return a * b / gcd(a, b);}
////////////////////////////////
//(x-r,y-r)-(x+r,y+r)
//(2*r+1)**2
bool kyori(vector<vector<int>>& wa,int h,int w,int x,int y,int r){
	if( x-r < 0 || y-r < 0 )return false;
	if( w <= x+r || h <= y+r )return false;
	int sum=0;
	sum = wa[y+r][x+r];
	int X,Y;
	X = x-r-1;
	Y = y-r-1;
	if( X >=0 && Y >= 0 ){
		sum += wa[Y][X];
	}
	if( X >= 0 ){sum -= wa[y+r][X];}
	if( Y >= 0 ){sum -= wa[Y][x+r];}
	return sum == (2*r+1)*(2*r+1);
}
inline void solve(){
	int H,W;
	scanf("%d%d",&H,&W);
	
	char str[3001];
	//累積和
	vector<vector<int>> wa(H,vector<int>(W,0));
	for(int i=0;i<H;++i){
		scanf(" %s",str);
		wa[i][0] = str[0] == '#';
		for(int j=1;j<W;++j){
			wa[i][j] = wa[i][j-1] + (str[j] == '#');
		}
	}
	
	//h方向
	for(int w=0;w<W;++w){
		for(int h=1;h<H;++h){
			wa[h][w] += wa[h-1][w];
		}
	}

	//////////////////////////////
	int maxLen = 1;
	for(int i=maxLen;i<H-maxLen;++i){
		for(int j=maxLen;j<W-maxLen;++j){
			//riku[i].first second
			while( kyori(wa,H,W, i, j,maxLen) == true ){
				++maxLen;
			}
		}
	}
	printf("%d",maxLen);
	return;
}

int main(void){
	//std::cin.tie(0);
	//std::ios::sync_with_stdio(false);
	//std::cout << std::fixed;//小数を10進数表示
	//cout << setprecision(16);//小数をいっぱい表示する。16?

	solve();

	return 0;
}