#include <bits/stdc++.h>

using namespace std;
using ll = long long;

int H, W;
vector<vector<int>> v;
vector<string> S;

bool solve(int X){
    vector<vector<int>> w(H+2, vector<int>(W+2));
    for (int i=1; i<=H-X+1; i++){
        for (int j=1; j<=W-X+1; j++){
            if (v[i+X-1][j+X-1]-v[i+X-1][j-1]-v[i-1][j+X-1]+v[i-1][j-1] == X*X){
                w[i][j]++; w[i+X][j]--; w[i][j+X]--; w[i+X][j+X]++;
            }
        }
    }

    for (int i=0; i<=H; i++){
        for (int j=1; j<=W; j++){
            w[i][j] += w[i][j-1];
        }
    }

    for (int i=1; i<=H; i++){
        for (int j=0; j<=W; j++){
            w[i][j] += w[i-1][j];
        }
    }

    for (int i=1; i<=H; i++){
        for (int j=1; j<=W; j++){
            if (S[i-1][j-1] == '#' && w[i][j] == 0) return 0;
        }
    }
    return 1;
}

int main(){

    cin >> H >> W;
    S.resize(H);
    for (int i=0; i<H; i++) cin >> S[i];
    v.resize(H+1, vector<int>(W+1));

    for (int i=1; i<=H; i++){
        for (int j=1; j<=W; j++){
            if (S[i-1][j-1] == '#') v[i][j]++;
            v[i][j] += v[i-1][j] + v[i][j-1] - v[i-1][j-1];
        }
    }

    int l=1, r=min(H, W)+1, c;
    while(r-l>1){
        c = (r+l)/2;
        if (solve(c)) l=c;
        else r=c;
    }

    cout << l << endl;

    return 0;
}