#include <bits/stdc++.h>
using namespace std;
using ll = long long;

template<class T> istream& operator >> (istream& is, vector<T>& vec) {
    for(T& x : vec) is >> x;
    return is;
}

template<class T> ostream& operator << (ostream& os, const vector<T>& vec) {
    if(vec.empty()) return os;
    os << vec[0];
    for(auto it = vec.begin(); ++it != vec.end(); ) os << ' ' << *it;
    return os;
}

template <class T> struct CumulativeSum2D{
    int h, w;
    std::vector<std::vector<T>> dat;
    CumulativeSum2D(int H, int W) : h(H), w(W), dat(H + 1, std::vector<T>(W + 1, 0)) {}
    CumulativeSum2D(std::vector<std::vector<T>> &A) 
        : h(A.size()), w(A[0].size()), dat(h + 1, std::vector<T>(w + 1, 0)) {
        for(int y = 1; y <= h; y++){
            for(int x = 1; x <= w; x++){
                dat[y][x] = A[y - 1][x - 1] + dat[y][x - 1] + dat[y - 1][x] - dat[y - 1][x - 1];
            }
        }
    }
    void add(int y, int x, T z){
        assert(0 <= y && y < h);
        assert(0 <= x && x < w);
        dat[y + 1][x + 1] += z;
    }
    void build(){
        for(int y = 1; y <= h; y++) {
            for(int x = 1; x <= w; x++) {
                dat[y][x] += dat[y][x - 1] + dat[y - 1][x] - dat[y - 1][x - 1];
            }
        }
    }
    T query(int ly, int lx, int ry, int rx){
        assert(0 <= ly && ly <= ry && ry <= h);
        assert(0 <= lx && lx <= rx && rx <= w);
        return dat[ry][rx] - dat[ly][rx] - dat[ry][lx] + dat[ly][lx];
    }
};

template <class T> struct imos2D{
    int h, w;
    std::vector<std::vector<T>> dat;
    imos2D(int H, int W) : h(H), w(W), dat(H + 1, std::vector<T>(W + 1, 0)) {}
    void add(int ly, int lx, int ry, int rx, T v){
        assert(0 <= ly && ly <= ry && ry <= h);
        assert(0 <= lx && lx <= rx && rx <= w);
        dat[ry][rx] += v;
        dat[ly][rx] -= v;
        dat[ry][lx] -= v;
        dat[ly][lx] += v;
    }
    void build(){
        for(int i = 0; i <= h; i++) {
            for(int j = 1; j <= w; j++) {
                dat[i][j] += dat[i][j - 1];
            }
        }
        for(int i = 0; i <= w; i++) {
            for(int j = 1; j <= h; j++) {
                dat[j][i] += dat[j - 1][i];
            }
        }
    }
    const std::vector<T>& operator[](int y) const {
        assert(0 <= y && y < h);
        return dat[y];
    }
    std::vector<T>& operator[](int y) { 
        assert(0 <= y && y < h);
        return dat[y];
    }
};

int main(){
    ios::sync_with_stdio(false);
    cin.tie(0);
    int h, w;
    cin >> h >> w;
    vector<string> A(h);
    cin >> A;
    CumulativeSum2D<int> S(h, w);
    for(int y = 0; y < h; y++){
        for(int x = 0; x < w; x++){
            if(A[y][x] == '#') S.add(y, x, 1);
        }
    }
    auto f = [&](int L){
        imos2D<int> S2(h, w);
        for(int y = 0; y + L <= h; y++){
            for(int x = 0; x + L <= w; x++){
                if(S.query(y, x, y + L, x + L) == L * L){
                    S2.add(y, x, y + L, x + L, 1);
                }
            }
        }
        S2.build();
        for(int y = 0; y < h; y++){
            for(int x = 0; x < w; x++){
                if(A[y][x] == '#' && S2[y][x] == 0) return false;
            }
        }
        return true;
    };
    S.build();
    int ok = 1, ng = min(h, w) + 1, mid;
    while(ok + 1 < ng){
        mid = (ok + ng) / 2;
        if(f(mid)) ok = mid;
        else ng = mid;
    }
    cout << ok << '\n';
}