#include <bits/stdc++.h>
using namespace std;
#define rep(i, n) for (int i = 0; i < (n); i++)
#define per(i, n) for (int i = (n)-1; i >= 0; i--)
#define rep2(i, l, r) for (int i = (l); i < (r); i++)
#define per2(i, l, r) for (int i = (r)-1; i >= (l); i--)
#define each(e, v) for (auto &e : v)
#define MM << " " <<
#define pb push_back
#define eb emplace_back
#define all(x) begin(x), end(x)
#define rall(x) rbegin(x), rend(x)
#define sz(x) (int)x.size()
using ll = long long;
using pii = pair<int, int>;
using pil = pair<int, ll>;
using pli = pair<ll, int>;
using pll = pair<ll, ll>;

template <typename T>
using minheap = priority_queue<T, vector<T>, greater<T>>;

template <typename T>
using maxheap = priority_queue<T>;

template <typename T>
bool chmax(T &x, const T &y) {
    return (x < y) ? (x = y, true) : false;
}

template <typename T>
bool chmin(T &x, const T &y) {
    return (x > y) ? (x = y, true) : false;
}

template <typename T>
int flg(T x, int i) {
    return (x >> i) & 1;
}

int pct(int x) { return __builtin_popcount(x); }
int pct(ll x) { return __builtin_popcountll(x); }
int topbit(int x) { return (x == 0 ? -1 : 31 - __builtin_clz(x)); }
int topbit(ll x) { return (x == 0 ? -1 : 63 - __builtin_clzll(x)); }
int botbit(int x) { return (x == 0 ? -1 : __builtin_ctz(x)); }
int botbit(ll x) { return (x == 0 ? -1 : __builtin_ctzll(x)); }

template <typename T>
void print(const vector<T> &v, T x = 0) {
    int n = v.size();
    for (int i = 0; i < n; i++) cout << v[i] + x << (i == n - 1 ? '\n' : ' ');
    if (v.empty()) cout << '\n';
}

template <typename T>
void printn(const vector<T> &v, T x = 0) {
    int n = v.size();
    for (int i = 0; i < n; i++) cout << v[i] + x << '\n';
}

template <typename T>
int lb(const vector<T> &v, T x) {
    return lower_bound(begin(v), end(v), x) - begin(v);
}

template <typename T>
int ub(const vector<T> &v, T x) {
    return upper_bound(begin(v), end(v), x) - begin(v);
}

template <typename T>
void rearrange(vector<T> &v) {
    sort(begin(v), end(v));
    v.erase(unique(begin(v), end(v)), end(v));
}

template <typename T>
vector<int> id_sort(const vector<T> &v, bool greater = false) {
    int n = v.size();
    vector<int> ret(n);
    iota(begin(ret), end(ret), 0);
    sort(begin(ret), end(ret), [&](int i, int j) { return greater ? v[i] > v[j] : v[i] < v[j]; });
    return ret;
}

template <typename T>
void reorder(vector<T> &a, const vector<int> &ord) {
    int n = a.size();
    vector<T> b(n);
    for (int i = 0; i < n; i++) b[i] = a[ord[i]];
    swap(a, b);
}

template <typename T>
T floor(T x, T y) {
    assert(y != 0);
    if (y < 0) x = -x, y = -y;
    return (x >= 0 ? x / y : (x - y + 1) / y);
}

template <typename T>
T ceil(T x, T y) {
    assert(y != 0);
    if (y < 0) x = -x, y = -y;
    return (x >= 0 ? (x + y - 1) / y : x / y);
}

template <typename S, typename T>
pair<S, T> operator+(const pair<S, T> &p, const pair<S, T> &q) {
    return make_pair(p.first + q.first, p.second + q.second);
}

template <typename S, typename T>
pair<S, T> operator-(const pair<S, T> &p, const pair<S, T> &q) {
    return make_pair(p.first - q.first, p.second - q.second);
}

template <typename S, typename T>
istream &operator>>(istream &is, pair<S, T> &p) {
    S a;
    T b;
    is >> a >> b;
    p = make_pair(a, b);
    return is;
}

template <typename S, typename T>
ostream &operator<<(ostream &os, const pair<S, T> &p) {
    return os << p.first << ' ' << p.second;
}

struct io_setup {
    io_setup() {
        ios_base::sync_with_stdio(false);
        cin.tie(NULL);
        cout << fixed << setprecision(15);
        cerr << fixed << setprecision(15);
    }
} io_setup;

constexpr int inf = (1 << 30) - 1;
constexpr ll INF = (1LL << 60) - 1;
// constexpr int MOD = 1000000007;
constexpr int MOD = 998244353;

template <typename T>
struct Cumulative_Sum_2D {
    vector<vector<T>> v;
    const int n, m;

    Cumulative_Sum_2D(const vector<vector<T>> &v) : v(v), n((int)v.size()), m((int)v[0].size()) {}

    Cumulative_Sum_2D(int n, int m) : v(n, vector<T>(m, 0)), n(n), m(m) {}

    // 累積和を構築
    void build() {
        for (int i = 0; i < n; i++) {
            for (int j = 1; j < m; j++) v[i][j] += v[i][j - 1];
        }
        for (int j = 0; j < m; j++) {
            for (int i = 1; i < n; i++) v[i][j] += v[i - 1][j];
        }
    }

    // 区間 [lx,rx) × [ly,ry) に imos 法で加算
    void add(int lx, int ly, int rx, int ry, T x) {
        lx = max(lx, 0), ly = max(ly, 0), rx = min(rx, n), ry = min(ry, m);
        if (rx <= lx || ry <= ly) return;
        v[lx][ly] += x;
        if (rx < n) v[rx][ly] -= x;
        if (ry < m) v[lx][ry] -= x;
        if (rx < n && ry < m) v[rx][ry] += x;
    }

    T fold(int a, int b) { return (a <= 0 || b <= 0 ? 0 : v[min(a, n) - 1][min(b, m) - 1]); }

    // 区間 [lx,rx) × [ly,ry) の総和
    T sum(int lx, int ly, int rx, int ry) {
        lx = max(lx, 0), ly = max(ly, 0), rx = min(rx, n), ry = min(ry, m);
        if (rx <= lx || ry <= ly) return 0;
        return fold(lx, ly) - fold(rx, ly) - fold(lx, ry) + fold(rx, ry);
    }
};

void solve() {
    int H, W;
    cin >> H >> W;

    vector<string> S(H);
    rep(i, H) cin >> S[i];

    vector<vector<int>> a(H, vector<int>(W));
    rep(i, H) rep(j, W) a[i][j] = (S[i][j] == '#' ? 1 : 0);

    Cumulative_Sum_2D<int> S1(a);
    S1.build();

    auto judge = [&](int K) {
        Cumulative_Sum_2D<int> S2(H, W);
        rep(i, H - K + 1) rep(j, W - K + 1) {
            if (S1.sum(i, j, i + K, j + K) == K * K) {
                S2.add(i, j, i + K, j + K, 1); //
            }
        }
        S2.build();
        rep(i, H) rep(j, W) {
            if (a[i][j] == 1 && S2.v[i][j] == 0) return false;
        }
        return true;
    };

    int ok = 1, ng = min(H, W) + 1;
    while (ng - ok > 1) {
        int mid = (ok + ng) / 2;
        (judge(mid) ? ok : ng) = mid;
    }

    cout << ok << '\n';
}

int main() {
    int T = 1;
    // cin >> T;
    while (T--) solve();
}