#include <algorithm>
#include <array>
#include <bitset>
#include <cassert>
#include <chrono>
#include <cmath>
#include <complex>
#include <deque>
#include <forward_list>
#include <fstream>
#include <functional>
#include <iomanip>
#include <ios>
#include <iostream>
#include <limits>
#include <list>
#include <map>
#include <memory>
#include <numeric>
#include <optional>
#include <queue>
#include <random>
#include <set>
#include <sstream>
#include <stack>
#include <string>
#include <tuple>
#include <type_traits>
#include <unordered_map>
#include <unordered_set>
#include <utility>
#include <vector>
using namespace std;
using lint = long long;
using pint = pair<int, int>;
using plint = pair<lint, lint>;
struct fast_ios {
    fast_ios() { cin.tie(nullptr), ios::sync_with_stdio(false), cout << fixed << setprecision(20); };
} fast_ios_;
#define ALL(x) (x).begin(), (x).end()
#define FOR(i, begin, end) for (int i = (begin), i##_end_ = (end); i < i##_end_; i++)
#define IFOR(i, begin, end) for (int i = (end)-1, i##_begin_ = (begin); i >= i##_begin_; i--)
#define REP(i, n) FOR(i, 0, n)
#define IREP(i, n) IFOR(i, 0, n)
template <typename T> bool chmax(T &m, const T q) { return m < q ? (m = q, true) : false; }
template <typename T> bool chmin(T &m, const T q) { return m > q ? (m = q, true) : false; }
const std::vector<std::pair<int, int>> grid_dxs{{1, 0}, {-1, 0}, {0, 1}, {0, -1}};
int floor_lg(long long x) { return x <= 0 ? -1 : 63 - __builtin_clzll(x); }
template <class T1, class T2> T1 floor_div(T1 num, T2 den) { return (num > 0 ? num / den : -((-num + den - 1) / den)); }
template <class T1, class T2> std::pair<T1, T2> operator+(const std::pair<T1, T2> &l, const std::pair<T1, T2> &r) { return std::make_pair(l.first + r.first, l.second + r.second); }
template <class T1, class T2> std::pair<T1, T2> operator-(const std::pair<T1, T2> &l, const std::pair<T1, T2> &r) { return std::make_pair(l.first - r.first, l.second - r.second); }
template <class T> std::vector<T> sort_unique(std::vector<T> vec) {
    sort(vec.begin(), vec.end()), vec.erase(unique(vec.begin(), vec.end()), vec.end());
    return vec;
}
template <class T> int arglb(const std::vector<T> &v, const T &x) { return std::distance(v.begin(), std::lower_bound(v.begin(), v.end(), x)); }
template <class T> int argub(const std::vector<T> &v, const T &x) { return std::distance(v.begin(), std::upper_bound(v.begin(), v.end(), x)); }
template <class IStream, class T> IStream &operator>>(IStream &is, std::vector<T> &vec) {
    for (auto &v : vec) is >> v;
    return is;
}

template <class OStream, class T> OStream &operator<<(OStream &os, const std::vector<T> &vec);
template <class OStream, class T, size_t sz> OStream &operator<<(OStream &os, const std::array<T, sz> &arr);
template <class OStream, class T, class TH> OStream &operator<<(OStream &os, const std::unordered_set<T, TH> &vec);
template <class OStream, class T, class U> OStream &operator<<(OStream &os, const pair<T, U> &pa);
template <class OStream, class T> OStream &operator<<(OStream &os, const std::deque<T> &vec);
template <class OStream, class T> OStream &operator<<(OStream &os, const std::set<T> &vec);
template <class OStream, class T> OStream &operator<<(OStream &os, const std::multiset<T> &vec);
template <class OStream, class T> OStream &operator<<(OStream &os, const std::unordered_multiset<T> &vec);
template <class OStream, class T, class U> OStream &operator<<(OStream &os, const std::pair<T, U> &pa);
template <class OStream, class TK, class TV> OStream &operator<<(OStream &os, const std::map<TK, TV> &mp);
template <class OStream, class TK, class TV, class TH> OStream &operator<<(OStream &os, const std::unordered_map<TK, TV, TH> &mp);
template <class OStream, class... T> OStream &operator<<(OStream &os, const std::tuple<T...> &tpl);

template <class OStream, class T> OStream &operator<<(OStream &os, const std::vector<T> &vec) {
    os << '[';
    for (auto v : vec) os << v << ',';
    os << ']';
    return os;
}
template <class OStream, class T, size_t sz> OStream &operator<<(OStream &os, const std::array<T, sz> &arr) {
    os << '[';
    for (auto v : arr) os << v << ',';
    os << ']';
    return os;
}
template <class... T> std::istream &operator>>(std::istream &is, std::tuple<T...> &tpl) {
    std::apply([&is](auto &&...args) { ((is >> args), ...); }, tpl);
    return is;
}
template <class OStream, class... T> OStream &operator<<(OStream &os, const std::tuple<T...> &tpl) {
    os << '(';
    std::apply([&os](auto &&...args) { ((os << args << ','), ...); }, tpl);
    return os << ')';
}
template <class OStream, class T, class TH> OStream &operator<<(OStream &os, const std::unordered_set<T, TH> &vec) {
    os << '{';
    for (auto v : vec) os << v << ',';
    os << '}';
    return os;
}
template <class OStream, class T> OStream &operator<<(OStream &os, const std::deque<T> &vec) {
    os << "deq[";
    for (auto v : vec) os << v << ',';
    os << ']';
    return os;
}
template <class OStream, class T> OStream &operator<<(OStream &os, const std::set<T> &vec) {
    os << '{';
    for (auto v : vec) os << v << ',';
    os << '}';
    return os;
}
template <class OStream, class T> OStream &operator<<(OStream &os, const std::multiset<T> &vec) {
    os << '{';
    for (auto v : vec) os << v << ',';
    os << '}';
    return os;
}
template <class OStream, class T> OStream &operator<<(OStream &os, const std::unordered_multiset<T> &vec) {
    os << '{';
    for (auto v : vec) os << v << ',';
    os << '}';
    return os;
}
template <class OStream, class T, class U> OStream &operator<<(OStream &os, const std::pair<T, U> &pa) { return os << '(' << pa.first << ',' << pa.second << ')'; }
template <class OStream, class TK, class TV> OStream &operator<<(OStream &os, const std::map<TK, TV> &mp) {
    os << '{';
    for (auto v : mp) os << v.first << "=>" << v.second << ',';
    os << '}';
    return os;
}
template <class OStream, class TK, class TV, class TH> OStream &operator<<(OStream &os, const std::unordered_map<TK, TV, TH> &mp) {
    os << '{';
    for (auto v : mp) os << v.first << "=>" << v.second << ',';
    os << '}';
    return os;
}
#ifdef HITONANODE_LOCAL
const string COLOR_RESET = "\033[0m", BRIGHT_GREEN = "\033[1;32m", BRIGHT_RED = "\033[1;31m", BRIGHT_CYAN = "\033[1;36m", NORMAL_CROSSED = "\033[0;9;37m", RED_BACKGROUND = "\033[1;41m", NORMAL_FAINT = "\033[0;2m";
#define dbg(x) std::cerr << BRIGHT_CYAN << #x << COLOR_RESET << " = " << (x) << NORMAL_FAINT << " (L" << __LINE__ << ") " << __FILE__ << COLOR_RESET << std::endl
#define dbgif(cond, x) ((cond) ? std::cerr << BRIGHT_CYAN << #x << COLOR_RESET << " = " << (x) << NORMAL_FAINT << " (L" << __LINE__ << ") " << __FILE__ << COLOR_RESET << std::endl : std::cerr)
#else
#define dbg(x) ((void)0)
#define dbgif(cond, x) ((void)0)
#endif

int main() {
    int H, W;
    cin >> H >> W;
    vector<string> S(H);
    cin >> S;

    vector cs(H + 1, vector<int>(W + 1));
    REP(i, H) REP(j, W) { cs.at(i + 1).at(j + 1) = cs.at(i + 1).at(j) + cs.at(i).at(j + 1) - cs.at(i).at(j) + (S.at(i).at(j) == '#'); }
    dbg(cs);

    int ok = 1, ng = min(H, W) + 1;
    while (ng - ok > 1) {
        const int c = (ok + ng) / 2;
        vector tmp(H + 1, vector<int>(W + 1));
        REP(i, H - c + 1) REP(j, W - c + 1) {
            int nin = cs.at(i + c).at(j + c) - cs.at(i).at(j + c) - cs.at(i + c).at(j) + cs.at(i).at(j);
            if (nin == c * c) {
                tmp.at(i).at(j)++;
                tmp.at(i + c).at(j + c)++;
                tmp.at(i).at(j + c)--;
                tmp.at(i + c).at(j)--;
            }
        }
        REP(i, H + 1) REP(j, W) tmp.at(i).at(j + 1) += tmp.at(i).at(j);
        REP(i, H) REP(j, W + 1) tmp.at(i + 1).at(j) += tmp.at(i).at(j);
        bool fail = false;
        REP(i, H) REP(j, W) {
            if (S.at(i).at(j) == '#' and !tmp.at(i).at(j)) fail = true;
        }
        (fail ? ng : ok) = c;
    }
    cout << ok << '\n';
}