#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

template <typename T, typename F>
struct SegmentTree2D {
    private:
        int id(int h, int w) { return h * 2 * W + w; }
    public:
        int H, W;
        vector<T> seg;
        const F f;
        const T I;
    SegmentTree2D(int h, int w, F _f, const T& i) : f(_f), I(i) { init(h, w); }

    void init(int h, int w) {
        H = W = 1;
        while (H < h) H <<= 1;
        while (W < w) W <<= 1;
        seg.assign(4 * H * W, I);
    }

    // build にのみ呼ぶ
    void set(int h, int w, const T& x) { seg[id(h + H, w + W)] = x; }

    void build() {
    // w in [W, 2W)
        for (int w = W; w < 2 * W; w++) {
            for (int h = H - 1; h; h--) {
                seg[id(h, w)] = f(seg[id(2 * h + 0, w)], seg[id(2 * h + 1, w)]);
            }
        }
    // h in [0, 2H)
        for (int h = 0; h < 2 * H; h++) {
            for (int w = W - 1; w; w--) {
                seg[id(h, w)] = f(seg[id(h, 2 * w + 0)], seg[id(h, 2 * w + 1)]);
            }
        }
    }

    T get(int h, int w)  { return seg[id(h + H, w + W)]; }
    // T operator()(int h, int w) const { return seg[id(h + H, w + W)]; }

    void update(int h, int w, const T& x) {
        h += H, w += W;
        seg[id(h, w)] = x;
        for (int i = h >> 1; i; i >>= 1) {
            seg[id(i, w)] = f(seg[id(2 * i + 0, w)], seg[id(2 * i + 1, w)]);
        }
        for (; h; h >>= 1) {
            for (int j = w >> 1; j; j >>= 1) {
                seg[id(h, j)] = f(seg[id(h, 2 * j + 0)], seg[id(h, 2 * j + 1)]);
            }
        }
    }

    T _inner_query(int h, int w1, int w2) {
        T res = I;
        for (; w1 < w2; w1 >>= 1, w2 >>= 1) {
            if (w1 & 1) res = f(res, seg[id(h, w1)]), w1++;
            if (w2 & 1) --w2, res = f(res, seg[id(h, w2)]);
        }
        return res;
    }

  // [ (h1,w1), (h2,w2) ) 半開
    T prod(int h1, int w1, int h2, int w2) {
        if (h1 >= h2 || w1 >= w2) return I;
        T res = I;
        h1 += H, h2 += H, w1 += W, w2 += W;
        for (; h1 < h2; h1 >>= 1, h2 >>= 1) {
            if (h1 & 1) res = f(res, _inner_query(h1, w1, w2)), h1++;
            if (h2 & 1) --h2, res = f(res, _inner_query(h2, w1, w2));
        }
        return res;
    }
};

/**
 * @brief 二次元セグメント木
 * @docs docs/data-structure-2d/2d-segment-tree.md
 * https://nyaannyaan.github.io/library/data-structure-2d/2d-segment-tree.hpp
 */
// 
int f(int a, int b){return a + b;}
int e(){return 0;}

int main(){
    cin.tie(0);
    ios_base::sync_with_stdio(false);
    int H,W; cin >> H >> W;
    vector<vector<char>> S(H, vector<char>(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;
        SegmentTree2D seg(H,W,f,e());
        seg.build();
        bool flag = true;
        rep(i,H)rep(j,W){
            int wa = seg.prod(max(i - k + 1, 0), max(j - k + 1, 0), i + 1, j + 1);
            if (S[i][j] == '.'){
                if (wa > 0) flag = false;
            }
            else{
                if (wa > 0) continue;
                else seg.update(i, j, 1);
            }
        }
        if (flag) ok = k;
        else ng = k;
    }
    cout << ok << endl;
}