#include <stdio.h>
#include <atcoder/all>
#include <bits/stdc++.h>
using namespace std;
using namespace atcoder;
using mint = modint998244353;
#define rep(i,n) for (int i = 0; i < (n); ++i)
#define Inf32 1000000001
#define Inf64 4000000000000000001

void Sum(vector<vector<int>> &sum){
	int h = sum.size()-1,w = sum[0].size()-1;
	rep(i,h){
		rep(j,w+1)sum[i+1][j] += sum[i][j];
	}
	rep(i,h+1){
		rep(j,w)sum[i][j+1] += sum[i][j];
	}
	
}

int main(){
	
	int h,w;
	cin>>h>>w;
	vector<string> s(h);
	rep(i,h)cin>>s[i];
	
	vector sum(h+1,vector<int>(w+1));
	rep(i,h){
		rep(j,w){
			if(s[i][j]=='#')sum[i+1][j+1] = 1;
		}
	}
	Sum(sum);
	
	int ok = 1,ng = max(h,w)+1;
	while(ng-ok>1){
		int mid = (ok+ng)/2;
		vector dp(h+1,vector<int>(w+1));
		rep(i,h){
			rep(j,w){
				if(i+mid>h||j+mid>w)continue;
				if(sum[i+mid][j+mid] - sum[i+mid][j] - sum[i][j+mid] + sum[i][j] == mid*mid){
					dp[i][j] ++;
					dp[i+mid][j] --;
					dp[i][j+mid] --;
					dp[i+mid][j+mid] ++;
					
				}
				
			}
		}
		Sum(dp);
		bool f  =true;
		rep(i,h){
			rep(j,w){
				if(s[i][j]=='#'&&dp[i][j]==0){
					f = false;
				}
			}
		}
		
		if(f)ok = mid;
		else ng = mid;
		
	}
	
	cout<<ok<<endl;

		
	return 0;
}