//#pragma GCC target("sse,sse2,sse3,ssse3,sse4,popcnt,abm,mmx,avx,avx2,avx512f")
#pragma GCC target("sse,sse2,sse3,ssse3,sse4,popcnt,abm,mmx,avx")
#pragma GCC optimize("O3")
#pragma GCC optimize("unroll-loops")
#include <iostream>
#include <iomanip>
#include <string>
#include <cmath>
#include <algorithm>
#include <vector>
#include <set>
#include <map>
#include <unordered_map>
#include <unordered_set>
#include <list>
#include <stack>
#include <queue>
#include <bitset>
#include <numeric>
#include <cassert>
#include <memory>
#include <random>
#include <functional>
#include <complex>
#include <immintrin.h>
#include <stdexcept>
#ifdef DEBUG
#include "./CompetitiveProgrammingCpp/Utils/debug.hpp"
#include "./CompetitiveProgrammingCpp/Utils/Timer.hpp"
#include "./CompetitiveProgrammingCpp/Utils/sample.hpp"
#else
#define dump(...)
template<class T>constexpr inline auto d_val(T a, T b) { return a; }
#endif

/* macro */
#define FOR(i, b, e) for(ll i = (ll)(b); i < (ll)(e); ++i)
#define RFOR(i, b, e) for(ll i = (ll)((e)-1); i >= (ll)(b); --i)
#define REP(i, n) FOR(i, 0, (n))
#define RREP(i, n) RFOR(i, 0, (n))
#define REPC(x,c) for(const auto& x:(c))
#define REPI2(it,b,e) for(auto it = (b); it != (e); ++it)
#define REPI(it,c) REPI2(it, (c).begin(), (c).end())
#define RREPI(it,c) REPI2(it, (c).rbegin(), (c).rend())
#define REPI_ERACE2(it, b, e) for(auto it = (b); it != (e);)
#define REPI_ERACE(it, c) REPI_ERACE2(it, (c).begin(), (c).end())
#define ALL(x) (x).begin(),(x).end()
/* macro func */
template<class T>
inline auto sort(T& t) { std::sort(ALL(t)); }
template<class T>
inline auto rsort(T& t) { std::sort((t).rbegin(), (t).rend()); }
template<class T, class S>
inline auto chmax(T& t, const S& s) { if(s > t) { t = s; return true; } return false; }
template<class T, class S>
inline auto chmin(T& t, const S& s) { if(s < t) { t = s; return true; } return false; }
inline auto BR() { std::cout << "\n"; }

/* type define */
using ll = long long;
template<class T>
using V = std::vector<T>;
using VS = V<std::string>;
using VL = V<ll>;
using VVL = V<VL>;
template<class T = ll, class U = T>
using P = std::pair<T, U>;
using PAIR = P<ll>;

/* using std */
using std::cout;
using std::cin;
using std::cerr;
constexpr char endl = '\n';

/* Initial processing  */
struct Preprocessing { Preprocessing() { std::cin.tie(0); std::ios::sync_with_stdio(0); }; }_Preprocessing;

/* define hash */
constexpr unsigned int static_random() { return 1u * __TIME__[0] * __TIME__[1] * __TIME__[3] * __TIME__[4] * __TIME__[6] * __TIME__[7]; }
template<class T> struct Hash { auto operator()(T x) const { return std::hash<T>()(x) ^ static_random(); } };
template<> struct Hash<std::pair<int, int>> { auto operator()(const std::pair<int, int>& x) const { return Hash<long long>()(x.first << 31 | x.second); } };

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

/* constant value */
// constexpr ll MOD = 1000000007;
constexpr ll MOD = 998244353;

//=============================================================================================


template <class Lambda>
auto binarySearch(long long mn, long long mx, const Lambda& judge, bool rev = false) {
    auto ok = mx + rev;
    auto ng = mn - 1;
    while(ok - ng > 1) {
        auto mid = (ok + ng) / 2;
        auto isOk = judge(mid);
        if((isOk && !rev) || (!isOk && rev)) {
            ok = mid;
        } else {
            ng = mid;
        }
    }
    return ok - rev;
}

template<
    class S,   // 要素の型
    S element, // 元
    class T, // 2項演算子
    class U // 逆元
>
struct Group {
    S m_val;
    Group() :m_val(element) {}
    Group(S val) :m_val(val) {}
    Group inverse()const { return U()(m_val); }
    Group binaryOperation(const Group& g)const { return T()(m_val, g.m_val); }
};

template<class P> struct F_A_Inv { auto operator()(P x)const { return -x; } };
template<class P> struct F_A_Bin { auto operator()(P x, P y)const { return x + y; } };
template<class P> using AdditiveGroup = Group<P, P(0), F_A_Bin<P>, F_A_Inv<P>>;

template <class Group = AdditiveGroup<long long>>
class Accumulation2D {
private:
    using S = decltype(Group().m_val);

    const int h;
    const int w;
    std::vector<std::vector<Group>> sumList;
public:

    Accumulation2D() = delete;
    Accumulation2D(const std::vector<std::vector<S>>& v) :
        h(v.size()),
        w(v[0].size()),
        sumList(h + 1, std::vector<Group>(w + 1)) {
        for(int i = 0; i < h; ++i) for(int j = 0; j < w; ++j) {
            sumList[i + 1][j + 1] = v[i][j];
        }
        for(int i = 0; i < h; ++i) for(int j = 0; j < w + 1; ++j) {
            sumList[i + 1][j] = sumList[i + 1][j].binaryOperation(sumList[i][j]);
        }
        for(int i = 0; i < h + 1; ++i) for(int j = 0; j < w; ++j) {
            sumList[i][j + 1] = sumList[i][j + 1].binaryOperation(sumList[i][j]);
        }
    }
    S get(int y, int x) {
        return sumList[y + 1][x + 1].m_val;
    }
    S get(int y1, int x1, int y2, int x2) {
        if(y2 < y1 || x2 < x1) { return Group().m_val; }
        x1 = std::max(x1, 0); y1 = std::max(y1, 0);
        y2 = std::min(y2, h - 1); x2 = std::min(x2, w - 1);
        return sumList[y2 + 1][x2 + 1]
            .binaryOperation(sumList[y1][x1])
            .binaryOperation(sumList[y2 + 1][x1].inverse())
            .binaryOperation(sumList[y1][x2 + 1].inverse()).m_val;
    }
};

signed main() {
    ll h, w;
    cin >> h >> w;
    VS s(h);
    cin >> s;
    VVL v(h, VL(w));
    REP(i, h)REP(j, w) { v[i][j] = s[i][j] == '#'; }
    auto acc = Accumulation2D<>(v);

    auto ans = binarySearch(1, std::max(h, w), [&](ll mid) {
        VVL imos(h, VL(w));
        ll mul = mid * mid;
        REP(i, h)REP(j, w) if(acc.get(i, j, i + mid - 1, j + mid - 1) == mul) {
            ++imos[i][j];
            if(i + mid < h) { --imos[i + mid][j]; }
            if(j + mid < w) { --imos[i][j + mid]; }
            if(i + mid < h && j + mid < w) { ++imos[i + mid][j + mid]; }
        }
        REP(i, h)REP(j, w - 1) { imos[i][j + 1] += imos[i][j]; }
        REP(i, h - 1)REP(j, w) { imos[i + 1][j] += imos[i][j]; }
        REP(i, h)REP(j, w) { chmin(imos[i][j], 1); }
        dump(mid, imos);
        return v == imos;
    }, true);
    cout << ans << endl;
}