#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

/* BIT2D:
    初期値は全て 0
    ・add(h,w,x): (h,w) に x を加算する
    ・sum(h,w): 1≦i≦h かつ 1≦j≦w の範囲の合計値を求める
    ・query(h1,w1,h2,w2): h1≦i<h2 かつ w1≦j<w2 の範囲の合計値を求める(1-indexed)
    計算量は全て O(logW * logH)
*/
template <typename T>
struct BIT2D {
    int H, W;
    vector<vector<T>> bit;  // データの格納先
    BIT2D(int H_, int W_) { init(H_, W_); }
    void init(int H_, int W_) {
        H = H_ + 1;
        W = W_ + 1;
        bit.assign(H, vector<T>(W, 0));
    }

    void add(int h, int w, T x) {
        for (int i = h; i < H; i += (i & -i)) {
            for (int j = w; j < W; j += (j & -j)) {
                bit[i][j] += x;
            }
        }
    }
    // 1≦i≦h かつ 1≦j≦w
    T sum(int h, int w) {
        T s(0);
        for (int i = h; i > 0; i -= (i & -i)) {
            for (int j = w; j > 0; j -= (j & -j)) {
                s += bit[i][j];
            }
        }
        return s;
    }

    // h1≦i<h2 かつ w1≦j<w2
    T query(int h1, int w1, int h2, int w2) {
        return sum(h2 - 1, w2 - 1) - sum(h2 - 1, w1 - 1) - sum(h1 - 1, w2 - 1) + sum(h1 - 1, w1 - 1);
    }
};

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

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

int main(){
    cin.tie(0);
    ios_base::sync_with_stdio(false);
    cin >> H >> W;
    for(int i=1;i<=H;i++){
        for(int j=1;j<=W;j++){
            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;
}