#ifdef LOCAL
//#define _GLIBCXX_DEBUG
#else
#pragma GCC optimize("O3")
#pragma GCC optimize("unroll-loops")
//#pragma GCC target("avx512f,avx512dq,avx512cd,avx512bw,avx512vl")
#endif

#include <bits/stdc++.h>
using namespace std;

typedef long long ll;
typedef unsigned long long ull;
typedef long double ld;
typedef pair<ll, ll> P;
typedef pair<int, int> Pi;
typedef vector<ll> Vec;
typedef vector<int> Vi;
typedef vector<string> Vs;
typedef vector<char> Vc;
typedef vector<P> VP;
typedef vector<VP> VVP;
typedef vector<Vec> VV;
typedef vector<Vi> VVi;
typedef vector<Vc> VVc;
typedef vector<VV> VVV;
typedef vector<VVV> VVVV;
#define MAKEVV(variable, a, ...) VV variable(a, Vec(__VA_ARGS__))
#define MAKEVVc(variable, a, ...) VVc variable(a,Vc(__VA_ARGS__))
#define MAKEVVV(variable, a, b, ...) VVV variable(a, VV(b, Vec(__VA_ARGS__)))
#define MAKEVVVV(variable, a, b, c, ...) VVVV variable(a, VVV(b, (VV(c, Vec(__VA_ARGS__)))))

#define endl '\n'
#define REP(i, a, b) for(ll i=(a); i<(b); i++)
#define PER(i, a, b) for(ll i=(a); i>=(b); i--)
#define rep(i, n) REP(i, 0, n)
#define per(i, n) PER(i, n, 0)
const ll INF = 4'000'000'000'000'000'010LL;
const ll MOD=998244353;
#define Yes(n) cout << ((n) ? "Yes" : "No") << endl;
#define YES(n) cout << ((n) ? "YES" : "NO") << endl;
#define ALL(v) v.begin(), v.end()
#define rALL(v) v.rbegin(), v.rend()
#define pb(x) push_back(x)
#define mp(a, b) make_pair(a,b)
#define Each(a,b) for(auto &a :b)
#define rEach(i, mp) for (auto i = mp.rbegin(); i != mp.rend(); ++i)
#define SUM(a) accumulate(ALL(a),0LL)
#define outminusone(a) cout<< ( a==INF ? -1 : a ) <<endl
#define Uniq(v) v.erase(unique(v.begin(), v.end()), v.end())
#define fi first
#define se second

template<class T, class S>bool chmax(T &a, const S &b) { if (a<b) { a=b; return true; } return false; }
template<class T, class S>bool chmin(T &a, const S &b) { if (b<a) { a=b; return true; } return false; }
template<class T>auto lb(vector<T> &X, T x){return lower_bound(ALL(X),x) - X.begin();}
template<class T>auto ub(vector<T> &X, T x){return upper_bound(ALL(X),x) - X.begin();}
ll popcnt(ll x){return __builtin_popcount(x);}
ll topbit(ll t){return t==0?-1:63-__builtin_clzll(t);}
ll floor(ll y,ll x){assert(x != 0);if(x < 0){y *= -1; x *= -1;}if(y < 0){return (y-x+1)/x;}return y/x;};
ll ceil(ll y, ll x){assert(x != 0);if(x < 0){y *= -1; x *= -1;}if(y < 0){return y/x;}return (y+x-1)/x;};

template<typename T1, typename T2>istream &operator>>(istream &i, pair<T1, T2> &p) { return i>>p.first>>p.second; }
template<typename T>istream& operator>>(istream&i,vector<T>&v){rep(j,v.size())i>>v[j];return i;}
template<typename T1, typename T2>ostream &operator<<(ostream &s, const pair<T1, T2> &p) { return s<<"("<<p.first<<", "<<p.second<<")"; }
template<class T>ostream &operator<<(ostream &os, const vector<T> &v) {bool f = false;for(const auto &d: v) {if(f) os<<" ";f = true;os<<d;}return os;}
template <class T> ostream& operator<<(ostream& os, const set<T>& s) {os << "{";bool f = false;for (auto d : s) {if (f) os << ", ";f = true;os << d;}return os << "}";}
template <class T> ostream& operator<<(ostream& os, const multiset<T>& s) {os << "{";bool f = false;for (auto d : s) {if (f) os << ", ";f = true;os << d;}return os << "}";}
template<class T, class U>ostream &operator<<(ostream &os, const map<T, U> &s) {bool f = false;os<<endl;for(auto p: s) {if(f) os<<endl;f = true;os<<p.first<<": "<<p.second;}return os<<endl;}
void out() { cout << endl; }
template <class Head, class... Tail> void out(const Head &head, const Tail &...tail) {cout << head;if(sizeof...(tail)) cout << ' ';out(tail...);}

#ifdef LOCAL
template<typename T>ostream &operator<<(ostream &s, const vector<vector<T>> &vv) {int len=vv.size();for(int i=0; i<len; ++i) {if(i==0)s<<endl;s<<i<<":"<<vv[i];if(i!=len-1)s<<endl;}return s;}
struct PrettyOS {ostream& os;bool first;template <class T> auto operator<<(T&& x) {if (!first) os << ", ";first = false;os << x;return *this;}};
template <class... T> void dbg0(T&&... t) {(PrettyOS{cerr, true} << ... << t);}
#define dbg(...)do {cerr << #__VA_ARGS__ << ":  ";dbg0(__VA_ARGS__);cerr << endl;} while (false);
#else
#define dbg(...)
#endif


template<class T>
struct CumulativeSum2D {
    vector<vector<T>> data;

    CumulativeSum2D(int H, int W) : data(H+1, vector<T>(W+1, 0)) {}
    void add(int y, int x, T z) {
        ++y, ++x;
        if(y >= data.size() || x >= data[0].size()) return;
        data[y][x] += z;
    }
    void build() {
        for(int i = 1; i < data.size(); i++) {
            for(int j = 1; j < data[i].size(); j++) {
                data[i][j] += data[i][j-1]+data[i-1][j]-data[i-1][j-1];
            }
        }
    }

    //gx gyは半開 gx, gyは含まないことに注意
    T query(int sy, int sx, int gy, int gx) {
        return (data[gy][gx]-data[sy][gx]-data[gy][sx]+data[sy][sx]);
    }

    void debug() {
        for(int i = 1; i < data.size(); i++) {
            dbg(data[i]);
        }
    }
};

int solve(){
    ll h,w;
    cin>>h>>w;
    VV g(h,Vec(w));
    Vs vs(h);
    cin>>vs;
    CumulativeSum2D<ll> cum(h,w);
    rep(i,h){
        rep(j,w){
            if(vs[i][j] == '#'){
                g[i][j] = 1;
                cum.add(i,j,1);
            }
        }
    }
    auto nibutan = [&](ll ok, ll ng, auto f)->ll {
        while (abs(ok-ng) > 1) {
            ll mid = (ok+ng)/2;
            tie(ok, ng) = (f(mid) ? mp(mid, ng) : mp(ok, mid));
        }
        return ok;
    };
    cum.build();

    auto canStamp = [&](ll i, ll j, ll k) -> bool{
        if(i+k > h)return false;
        if(j+k > w)return false;
        return cum.query(i,j,i+k,j+k) == k * k;
    };
    auto judge = [&](ll k)->bool{
        VV imos(h+1,Vec(w+1));
        rep(i,h){
            rep(j,w){
                if(canStamp(i,j,k)){
                    imos[i][j]++;
                    imos[i+k][j]--;
                    imos[i][j+k]--;
                    imos[i+k][j+k]++;
                }
            }
        }
        rep(i,h+1){
            rep(j,w){
                imos[i][j+1] += imos[i][j];
            }
        }
        rep(i,h){
            rep(j,w+1){
                imos[i+1][j] += imos[i][j];
            }
        }
        bool ok = true;
        rep(i,h){
            rep(j,w){
                if(g[i][j] and imos[i][j] == 0)ok = false;
            }
        }
        dbg(k)
        dbg(imos)
        dbg(g)
        return ok;
    };
    ll ok = nibutan(0,min(h,w)+1,judge);
    out(ok);
    return  0;
}

int main() {
    cin.tie(nullptr);
    ios::sync_with_stdio(false);
    cout<<std::setprecision(20);
//    ll T;
//    cin>>T;
//    while(T--)
    solve();
}