#if __INCLUDE_LEVEL__ == 0

#include <bits/stdc++.h>

using namespace std;

#include __BASE_FILE__

namespace {

void solve() {
  int h, w;
  cin >> tie(h, w);
  vector<string> s(h);
  cin >> s;
  vector suff(h + 1, vector<int>(w + 1));
  for (int i : rep(h)) {
    for (int j : per(w)) {
      suff[i][j] = (s[i][j] == '#') + suff[i][j + 1];
    }
  }
  for (int j : rep(w)) {
    for (int i : per(h)) {
      suff[i][j] += suff[i + 1][j];
    }
  }
  auto check = [&](int m) -> bool {
    vector f(h + 1, vector<int>(w + 1));
    for (int i : rep1(0, h - m)) {
      for (int j : rep1(0, w - m)) {
        int c = suff[i][j];
        c -= suff[i][j + m];
        c -= suff[i + m][j];
        c += suff[i + m][j + m];
        if (c == m * m) {
          ++f[i][j];
          --f[i][j + m];
          --f[i + m][j];
          ++f[i + m][j + m];
        }
      }
    }
    for (int i : rep(h)) {
      for (int j : rep(w)) {
        f[i][j + 1] += f[i][j];
      }
    }
    for (int j : rep(w)) {
      for (int i : rep(h)) {
        f[i + 1][j] += f[i][j];
      }
    }
    for (int i : rep(h)) {
      for (int j : rep(w)) {
        if ((s[i][j] == '#') != (0 < f[i][j])) {
          return false;
        }
      }
    }
    return true;
  };
  int ok = 1;
  int ng = min(h, w) + 1;
  while (ok + 1 < ng) {
    int mid = midpoint(ok, ng);
    (check(mid) ? ok : ng) = mid;
  }
  print(ok);
}

}  // namespace

int main() {
  ios::sync_with_stdio(false);
  cin.tie(nullptr);

  solve();
}

#else  // __INCLUDE_LEVEL__

template <class T, class U = T>
bool chmin(T& x, U&& y) {
  return y < x && (x = forward<U>(y), true);
}

template <class T, class U = T>
bool chmax(T& x, U&& y) {
  return x < y && (x = forward<U>(y), true);
}

namespace std {

template <class T1, class T2>
istream& operator>>(istream& is, pair<T1, T2>& p) {
  return is >> p.first >> p.second;
}

template <class... Ts>
istream& operator>>(istream& is, tuple<Ts...>& t) {
  return apply([&is](auto&... xs) -> istream& { return (is >> ... >> xs); }, t);
}

template <class... Ts>
istream& operator>>(istream& is, tuple<Ts&...>&& t) {
  return is >> t;
}

template <class R, enable_if_t<!is_convertible_v<R, string>>* = nullptr>
auto operator>>(istream& is, R&& r) -> decltype(is >> *begin(r)) {
  for (auto&& e : r) {
    is >> e;
  }
  return is;
}

template <class T1, class T2>
ostream& operator<<(ostream& os, const pair<T1, T2>& p) {
  return os << p.first << ' ' << p.second;
}

template <class... Ts>
ostream& operator<<(ostream& os, const tuple<Ts...>& t) {
  auto f = [&os](const auto&... xs) -> ostream& {
    [[maybe_unused]] auto sep = "";
    ((os << exchange(sep, " ") << xs), ...);
    return os;
  };
  return apply(f, t);
}

template <class R, enable_if_t<!is_convertible_v<R, string_view>>* = nullptr>
auto operator<<(ostream& os, R&& r) -> decltype(os << *begin(r)) {
  auto sep = "";
  for (auto&& e : r) {
    os << exchange(sep, " ") << e;
  }
  return os;
}

}  // namespace std

template <class... Ts>
void print(Ts&&... xs) {
  cout << tie(xs...) << '\n';
}

inline auto rep(int l, int r) { return views::iota(min(l, r), r); }
inline auto rep(int n) { return rep(0, n); }
inline auto rep1(int l, int r) { return rep(l, r + 1); }
inline auto rep1(int n) { return rep(1, n + 1); }
inline auto per(int l, int r) { return rep(l, r) | views::reverse; }
inline auto per(int n) { return per(0, n); }
inline auto per1(int l, int r) { return per(l, r + 1); }
inline auto per1(int n) { return per(1, n + 1); }

#endif  // __INCLUDE_LEVEL__