#include <bits/stdc++.h>
#include <atcoder/all>
using namespace std;
using namespace atcoder;
istream &operator>>(istream &is, modint &a) { long long v; is >> v; a = v; return is; }
ostream &operator<<(ostream &os, const modint &a) { return os << a.val(); }
istream &operator>>(istream &is, modint998244353 &a) { long long v; is >> v; a = v; return is; }
ostream &operator<<(ostream &os, const modint998244353 &a) { return os << a.val(); }
istream &operator>>(istream &is, modint1000000007 &a) { long long v; is >> v; a = v; return is; }
ostream &operator<<(ostream &os, const modint1000000007 &a) { return os << a.val(); } 

typedef long long ll;
typedef vector<vector<int>> Graph;
typedef pair<int, int> pii;
typedef pair<ll, ll> pll;
#define rep(i,n) for (int i = 0;i < (int)(n); i++)
#define all(x) x.begin(), x.end()
#define rall(x) x.rbegin(), x.rend()
#define my_sort(x) sort(x.begin(), x.end())
#define my_max(x) *max_element(all(x))
#define my_min(x) *min_element(all(x))
template<class T> inline bool chmax(T& a, T b) { if (a < b) { a = b; return 1; } return 0; }
template<class T> inline bool chmin(T& a, T b) { if (a > b) { a = b; return 1; } return 0; }
const int INF = (1<<30) - 1;
const ll LINF = (1LL<<62) - 1;
const int MOD = 998244353;
const int MOD2 = 1e9+7;
const double PI = acos(-1);
vector<int> di = {1,0,-1,0};
vector<int> dj = {0,1,0,-1};

#ifdef LOCAL
#  include <debug_print.hpp>
#  define debug(...) debug_print::multi_print(#__VA_ARGS__, __VA_ARGS__)
#else
#  define debug(...) (static_cast<void>(0))
#endif

struct cumulative_sum_2d{
    int H,W;
    vector<vector<ll>> val;
    vector<vector<ll>> wa;
    cumulative_sum_2d(int H_input, int W_input){init(H_input,W_input);}
    void init(int H_input, int W_input){
        H = H_input;
        W = W_input;
        val.assign(H,vector<ll>(W,0ll));
        wa.assign(H+1,vector<ll>(W+1,0ll));
    }
    void set(int i, int j, ll value){
        val[i][j] = value;
    }
    void add(int i, int j, ll value){
        val[i][j] += value;
    }
    void build(){
        for(int i=0;i<H;i++){
            for(int j=0;j<W;j++){
                wa[i+1][j+1] = wa[i][j+1] + wa[i+1][j] - wa[i][j] + val[i][j];
            }
        }
    }
    // [i1, i2) x [j1, j2)
    ll query(int i1,int j1, int i2, int j2){
        return wa[i2][j2] - wa[i2][j1] - wa[i1][j2] + wa[i1][j1];
    }
};

template<typename T>
struct BIT2D{
    int H, W;
    vector<vector<T>> bit;
    BIT2D(int _H, int _W) : H(_H), W(_W){
        bit.resize(H, vector<T>(W, 0));
    }
    //
    void add(int i, int j, T v){
        for(int a = i + 1; a <= H; a += (a & -a)){
            for(int b = j + 1; b <= W; b += (b & - b)){
                bit[a - 1][b - 1] += v;
            }
        }
    }
    //
    T _sum(int i, int j){
        T s = 0;
        for(int a = i; a > 0; a -= (a & -a)){
            for(int b = j; b > 0; b -= (b & -b)){
                s += bit[a - 1][b - 1];
            }
        }
        return s;
    }
    //[i1, i2) x [j1, j2)
    T sum(int i1, int j1, int i2, int j2){
        return _sum(i2, j2) - _sum(i2, j1) - _sum(i1, j2) + _sum(i1, j1);
    }
};

int H,W;
char S[2020][2020];
int wa[2020][2020];

bool is_ok(int k){
    cumulative_sum_2d acc(H, W);
    rep(i,H)rep(j,W)if(S[i][j]=='#')acc.add(i,j,1);
    acc.build();
    BIT2D<int> BIT(H, W);
    rep(i,H-k+1)rep(j,W-k+1){
        if(acc.query(i,j,i+k,j+k) == (k * k)) BIT.add(i,j,1);
    }
    rep(i,H)rep(j,W){
        int wa = BIT.sum(max(i - k + 1, 0), max(j - k + 1, 0), i + 1, j + 1);
        if (S[i][j]=='.'){
            if (wa > 0) return false;
        }
        else{
            if (wa == 0) return false;
        }
    }
    return true;
}

int main(){
    cin.tie(0);
    ios_base::sync_with_stdio(false);
    cin >> H >> W;
    rep(i,H)rep(j,W) cin >> S[i][j];
    int ok = 0, ng = H + 1;
    while(abs(ok - ng) > 1){
        int k = (ok + ng) / 2;
        if (is_ok(k)) ok = k;
        else ng = k;
    }
    cout << ok << endl;
}