#include<bits/stdc++.h>
using namespace std;
#define rep(i,a,b) for(int i=a;i<(b);i++)
#define all(a) begin(a),end(a)
#define sz(a) (int)(a).size()
typedef long long ll;
typedef vector<int> vi;
typedef pair<int,int> pii;
int main(){
	cin.tie(0)->sync_with_stdio(0);
	cin.exceptions(cin.failbit);
	int H,W;cin>>H>>W;
	vector<string>S(H);
	rep(i,0,H)cin>>S[i];
	vector<vector<int>>A(H+1,vector<int>(W+1));
	rep(i,0,H)rep(j,0,W)A[i+1][j+1]=A[i+1][j]+A[i][j+1]-A[i][j]+(S[i][j]=='#');
	int l=1,r=min(H,W)+1;
	while(r-l>1){
		int m=(l+r)/2;
		vector<vector<int>>B(H+1,vector<int>(W+1));
		rep(i,0,H+1-m)rep(j,0,W+1-m){
			if(A[i][j]-A[i+m][j]-A[i][j+m]+A[i+m][j+m]==m*m){
				B[i][j]++;
				B[i+m][j]--;
				B[i][j+m]--;
				B[i+m][j+m]++;
			}
		}
		rep(i,0,H)rep(j,0,W){
			B[i+1][j]+=B[i][j];
			B[i][j+1]+=B[i][j];
			B[i+1][j+1]-=B[i][j];
		}
		bool ok=1;
		rep(i,0,H)rep(j,0,W)if(B[i][j]==0&&S[i][j]=='#')ok=0;
		if(ok)l=m;
		else r=m;
	}
	cout<<l<<endl;
}