#include #include 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> Graph; typedef pair pii; typedef pair 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 inline bool chmax(T& a, T b) { if (a < b) { a = b; return 1; } return 0; } template 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 di = {1,0,-1,0}; vector dj = {0,1,0,-1}; #ifdef LOCAL # include # define debug(...) debug_print::multi_print(#__VA_ARGS__, __VA_ARGS__) #else # define debug(...) (static_cast(0)) #endif struct cumulative_sum_2d{ int H,W; vector> val; vector> wa; cumulative_sum_2d(int H_input, int W_input){init(H_input,W_input);} void init(int H_input, int W_input){ H = H_input; W = W_input; val.assign(H,vector(W,0ll)); wa.assign(H+1,vector(W+1,0ll)); } void set(int i, int j, ll value){ val[i][j] = value; } void add(int i, int j, ll value){ val[i][j] += value; } void build(){ for(int i=0;i struct BIT2D{ int H, W; vector> bit; BIT2D(int _H, int _W) : H(_H), W(_W){ bit.resize(H, vector(W, 0)); } // void add(int i, int j, T v){ for(int a = i + 1; a <= H; a += (a & -a)){ for(int b = j + 1; b <= W; b += (b & - b)){ bit[a - 1][b - 1] += v; } } } // T _sum(int i, int j){ T s = 0; for(int a = i; a > 0; a -= (a & -a)){ for(int b = j; b > 0; b -= (b & -b)){ s += bit[a - 1][b - 1]; } } return s; } //[i1, i2) x [j1, j2) T sum(int i1, int j1, int i2, int j2){ return _sum(i2, j2) - _sum(i2, j1) - _sum(i1, j2) + _sum(i1, j1); } }; int H,W; char S[2020][2020]; int wa[2020][2020]; bool is_ok(int k){ cumulative_sum_2d acc(H, W); rep(i,H)rep(j,W)if(S[i][j]=='#')acc.add(i,j,1); acc.build(); BIT2D BIT(H, W); rep(i,H-k+1)rep(j,W-k+1){ if(acc.query(i,j,i+k,j+k) == (k * k)) BIT.add(i,j,1); } rep(i,H)rep(j,W){ int wa = BIT.sum(max(i - k + 1, 0), max(j - k + 1, 0), i + 1, j + 1); if (S[i][j]=='.'){ if (wa > 0) return false; } else{ if (wa == 0) return false; } } return true; } int main(){ cin.tie(0); ios_base::sync_with_stdio(false); cin >> H >> 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; if (is_ok(k)) ok = k; else ng = k; } cout << ok << endl; }