#include <atcoder/all>
using namespace atcoder;
using mint = modint998244353;
const long long MOD = 998244353;
// using mint = modint1000000007;
// const long long MOD = 1000000007;
// using mint = modint;//mint::set_mod(MOD);

#include <bits/stdc++.h>
#define rep(i, a, b) for (ll i = (ll)(a); i < (ll)(b); i++)
#define repeq(i, a, b) for (ll i = (ll)(a); i <= (ll)(b); i++)
#define repreq(i, a, b) for (ll i = (ll)(a); i >= (ll)(b); i--)
#define endl '\n'  // fflush(stdout);
#define cYes cout << "Yes" << endl
#define cNo cout << "No" << endl
#define sortr(v) sort(v, greater<>())
#define pb push_back
#define pob pop_back
#define mp make_pair
#define mt make_tuple
#define FI first
#define SE second
#define ALL(v) (v).begin(), (v).end()
#define INFLL 3000000000000000100LL
#define INF 1000000100
#define PI acos(-1.0L)
#define TAU (PI * 2.0L)

using namespace std;

typedef long long ll;
typedef pair<ll, ll> Pll;
typedef tuple<ll, ll, ll> Tlll;
typedef vector<int> Vi;
typedef vector<Vi> VVi;
typedef vector<ll> Vl;
typedef vector<Vl> VVl;
typedef vector<VVl> VVVl;
typedef vector<Tlll> VTlll;
typedef vector<mint> Vm;
typedef vector<Vm> VVm;
typedef vector<string> Vs;
typedef vector<double> Vd;
typedef vector<char> Vc;
typedef vector<bool> Vb;
typedef vector<Pll> VPll;
typedef priority_queue<ll> PQl;
typedef priority_queue<ll, vector<ll>, greater<ll>> PQlr;

/* inout */
ostream &operator<<(ostream &os, mint const &m) {
    os << m.val();
    return os;
}
istream &operator>>(istream &is, mint &m) {
    long long n;
    is >> n, m = n;
    return is;
}
template <typename T>
ostream &operator<<(ostream &os, const vector<T> &v) {
    int n = v.size();
    rep(i, 0, n) { os << v[i] << " \n"[i == n - 1]; }
    return os;
}
template <typename T>
ostream &operator<<(ostream &os, const vector<vector<T>> &v) {
    int n = v.size();
    rep(i, 0, n) os << v[i];
    return os;
}
template <typename T, typename S>
ostream &operator<<(ostream &os, pair<T, S> const &p) {
    os << p.first << ' ' << p.second;
    return os;
}
template <typename T, typename S>
ostream &operator<<(ostream &os, const map<T, S> &mp) {
    for (auto &[key, val] : mp) {
        os << key << ':' << val << '\n';
    }
    return os;
}
template <typename T>
ostream &operator<<(ostream &os, const set<T> &st) {
    auto itr = st.begin();
    for (int i = 0; i < (int)st.size(); i++) {
        os << *itr << (i + 1 != (int)st.size() ? ' ' : '\n');
        itr++;
    }
    return os;
}
template <typename T>
ostream &operator<<(ostream &os, multiset<T> &st) {
    auto itr = st.begin();
    for (int i = 0; i < (int)st.size(); i++) {
        os << *itr << (i + 1 != (int)st.size() ? ' ' : '\n');
        itr++;
    }
    return os;
}
template <typename T>
ostream &operator<<(ostream &os, queue<T> q) {
    while (q.size()) {
        os << q.front();
        q.pop();
        os << " \n"[q.empty()];
    }
    return os;
}
template <typename T>
ostream &operator<<(ostream &os, stack<T> st) {
    vector<T> v;
    while (st.size()) {
        v.push_back(st.top());
        st.pop();
    }
    reverse(ALL(v));
    os << v;
    return os;
}
template <class T, class Container, class Compare>
ostream &operator<<(ostream &os, priority_queue<T, Container, Compare> pq) {
    vector<T> v;
    while (pq.size()) {
        v.push_back(pq.top());
        pq.pop();
    }
    os << v;
    return os;
}
template <typename T>
istream &operator>>(istream &is, vector<T> &v) {
    for (T &in : v) is >> in;
    return is;
}
template <typename T1, typename T2>
istream &operator>>(istream &is, pair<T1, T2> &p) {
    is >> p.first >> p.second;
    return is;
}

/* useful */
template <typename T>
int SMALLER(vector<T> &a, T x) {
    return lower_bound(a.begin(), a.end(), x) - a.begin();
}
template <typename T>
int orSMALLER(vector<T> &a, T x) {
    return upper_bound(a.begin(), a.end(), x) - a.begin();
}
template <typename T>
int BIGGER(vector<T> &a, T x) {
    return a.size() - orSMALLER(a, x);
}
template <typename T>
int orBIGGER(vector<T> &a, T x) {
    return a.size() - SMALLER(a, x);
}
template <typename T>
int COUNT(vector<T> &a, T x) {
    return upper_bound(ALL(a), x) - lower_bound(ALL(a), x);
}
template <typename T, typename S>
bool chmax(T &a, S b) {
    if (a < b) {
        a = b;
        return 1;
    }
    return 0;
}
template <typename T, typename S>
bool chmin(T &a, S b) {
    if (a > b) {
        a = b;
        return 1;
    }
    return 0;
}
template <typename T>
void press(T &v) {
    v.erase(unique(ALL(v)), v.end());
}
template <typename T>
vector<int> zip(vector<T> b) {
    pair<T, int> p[b.size() + 10];
    int a = b.size();
    vector<int> l(a);
    for (int i = 0; i < a; i++) p[i] = mp(b[i], i);
    sort(p, p + a);
    int w = 0;
    for (int i = 0; i < a; i++) {
        if (i && p[i].first != p[i - 1].first) w++;
        l[p[i].second] = w;
    }
    return l;
}
template <typename T>
vector<T> vis(vector<T> &v) {
    vector<T> S(v.size() + 1);
    rep(i, 1, S.size()) S[i] += v[i - 1] + S[i - 1];
    return S;
}

ll dem(ll a, ll b) { return ((a + b - 1) / (b)); }
ll dtoll(double d, int g) { return round(d * pow(10, g)); }

const double EPS = 1e-10;

void init() {
    cin.tie(0);
    cout.tie(0);
    ios::sync_with_stdio(0);
    cout << fixed << setprecision(12);
}

// do {} while (next_permutation(ALL(vec)));

/********************************** START **********************************/

void sol();

int main() {
    init();
    int q = 1;
    // cin >> q;
    while (q--) sol();
    return 0;
}

/********************************** SOLVE **********************************/

template <typename T>
struct TDS {
    // 区間はすべて半開区間
    int h, w;
    bool rect = false;
    vector<vector<T>> v, vv, s;
    TDS() {}
    TDS(int h, int w) : h(h), w(w) {
        v = vector<vector<T>>(h, vector<T>(w));
        vv = vector<vector<T>>(h + 1, vector<T>(w + 1));
    }
    void add_point(int i, int j, T c) { v[i][j] += c; }
    void add_point(int li, int ri, int lj, int rj, T c) {
        rect |= true;
        vv[li][lj] += c;
        vv[li][rj] -= c;
        vv[ri][lj] -= c;
        vv[ri][rj] += c;
    }
    void make() {
        s = vector<vector<T>>(h + 1, vector<T>(w + 1));
        if (rect) {
            rect = false;
            vector ss = vector<vector<T>>(h, vector<T>(w));
            rep(i, 0, h) {
                rep(j, 0, w) {
                    ss[i][j] = vv[i][j];
                    if (j > 0) ss[i][j] += ss[i][j - 1];
                }
            }
            rep(j, 0, w) {
                rep(i, 1, h) { ss[i][j] += ss[i - 1][j]; }
            }
            rep(i, 0, h) rep(j, 0, w) add_point(i, j, ss[i][j]);
        }
        rep(i, 0, h) {
            rep(j, 0, w) { s[i + 1][j + 1] = s[i + 1][j] + v[i][j]; }
        }
        rep(j, 0, w) {
            rep(i, 0, h) { s[i + 1][j + 1] += s[i][j + 1]; }
        }
    }
    T sum(int li, int ri, int lj, int rj) {
        if (lj >= rj || li >= ri) return 0LL;
        T ret = s[ri][rj] - s[ri][lj] - s[li][rj] + s[li][lj];
        return ret;
    }
};

void sol() {
    int h, w;
    cin >> h >> w;
    TDS<int> pl1(h, w);
    Vs s(h);
    cin >> s;
    rep(i, 0, h) rep(j, 0, w) {
        if (s[i][j] == '#') pl1.add_point(i, j, 1);
    }
    pl1.make();
    auto check = [&](ll mid) {  // ([...,1,1,1,0,0,0,...])
        if (mid > h || mid > w) return 0;
        TDS<int> pl2(h, w);

        rep(i, 0, h - mid + 1) rep(j, 0, w - mid + 1) {
            if (pl1.sum(i, i + mid, j, j + mid) == mid * mid)
                pl2.add_point(i, i + mid, j, j + mid, 1);
        }

        Vs t(h, string(w, '.'));
        pl2.make();
        rep(i, 0, h) rep(j, 0, w) {
            if (pl2.sum(i, i + 1, j, j + 1)) t[i][j] = '#';
        }
        bool ok = (s == t);
        // cerr << mid << ' ' << ok << endl;
        // cerr << mid << endl;
        rep(i, 0, h) cerr << t[i] << endl;
        if (ok)
            return 1;
        else
            return 0;
    };
    auto binary = [&]() {  // ll
        ll L = 0, R = 1;   // 解[L,R) 探索範囲(L,R)
        while (check(R)) R *= 2;
        ll mid = L + (R - L) / 2;
        while (R - L > 1) {
            if (check(mid))
                L = mid;
            else
                R = mid;
            mid = L + (R - L) / 2;
        }
        return L;  // L-true R-false
    };
    cout << binary() << endl;
}