#include <bits/stdc++.h>
#pragma GCC optimize("O3")
using namespace std;
// define
#define endl "\n"
#ifdef LOCAL
#define lout cerr
#else
ostream devnull(0);
#define lout devnull
#endif
// rep
// Inspired by:
// https://github.com/kurokoji/.cpp-Template/blob/master/template/template2.cpp
#define GET_MACRO(_1, _2, _3, _4, NAME, ...) NAME
#define rep(...)                                 \
  GET_MACRO(__VA_ARGS__, rep4, rep3, rep2, rep1) \
  (__VA_ARGS__)
#define rep1(n) rep2(_, n)
#define rep2(i, n) rep3(i, 0, n)
#define rep3(i, a, n) rep4(i, a, n, 1)
#define rep4(i, a, n, d) rep5(i, a, n, d, <)
#define rep5(i, a, n, d, o) \
  for (auto i = decltype(n)(a), i##_len = (n); i o i##_len; i += (d))

#define rrep(...)                                    \
  GET_MACRO(__VA_ARGS__, rrep4, rrep3, rrep2, rrep1) \
  (__VA_ARGS__)
#define rrep1(a) rrep2(_, a)
#define rrep2(i, a) rrep3(i, a, 0)
#define rrep3(i, a, n) rrep4(i, a, n, -1)
#define rrep4(i, a, n, d) rep5(i, (a)-1, n, d, >=)

#define repc(...)                                    \
  GET_MACRO(__VA_ARGS__, repc4, repc3, repc2, repc1) \
  (__VA_ARGS__)
#define repc1(n) repc2(_, n)
#define repc2(i, n) repc3(i, 0, n)
#define repc3(i, a, n) repc4(i, a, n, 1)
#define repc4(i, a, n, d) rep5(i, a, n, d, <=)
// alias
// typedef long long ll;
// typedef long long lint;
// typedef pair<int, int> pii;
// typedef pair<lint, lint> pll;
using ll = long long;
using lint = long long;
// func
template <class C>
void vcin(C &c)
{
  for (auto &&a : c)
  {
    cin >> a;
  }
}
template <class C, class D>
void vcin(C &c, D &d)
{
  for (auto it1 = c.begin(), it2 = d.begin(); it1 != c.end() && it2 != d.end();
       it1++, it2++)
  {
    cin >> *it1 >> *it2;
  }
}
template <class C>
void vcout(const C &c, const string delimiter = " ")
{
  for (auto it = c.begin(); it != c.end(); it++)
  {
    if (it != c.begin())
      cout << delimiter;
    cout << *it;
  }
}
template <class T = int>
inline T input()
{
  T x;
  cin >> x;
  return (x);
}
template <typename T>
inline void chmax(T &x, const T y)
{
  if (x < y)
    x = y;
}
template <typename T>
inline void chmin(T &x, const T y)
{
  if (x > y)
    x = y;
}
#define CPPV (__cplusplus / 100 % 100) //C++ Version
#if CPPV >= 14

// http://beet-aizu.hatenablog.com/entry/2018/04/08/145516
template <typename T>
vector<T> make_v(size_t a) { return vector<T>(a); }
template <typename T, typename... Ts>
auto make_v(size_t a, Ts... ts)
{
  return vector<decltype(make_v<T>(ts...))>(a, make_v<T>(ts...));
}
template <typename T, typename V>
typename enable_if<is_class<T>::value == 0>::type fill_v(T &t, const V &v)
{
  t = v;
}
template <typename T, typename V>
typename enable_if<is_class<T>::value != 0>::type fill_v(T &t, const V &v)
{
  for (auto &e : t)
    fill_v(e, v);
}
#endif

constexpr int INF = 1 << 24;
constexpr lint LINF = 1LL << 48;

#define dump(x) lout << #x << " = " << (x) << endl;
inline lint bit(lint n)
{
  return 1LL << n;
}

void solve();

signed main()
{
  cin.tie(0);
  ios::sync_with_stdio(false);
  cout << fixed << setprecision(15);

  solve();

  return 0;
}

void solve()
{
  int h, w;
  cin >> h >> w;
  auto s = make_v<char>(h, w);
  rep(i, h) rep(j, w)
  {
    cin >> s[i][j];
  }

  auto d = make_v<int>(h * w, h * w);
  fill_v(d, INF);

  vector<int> dx = {1, 0, -1, 0};
  vector<int> dy = {0, 1, 0, -1};

  auto f = [&](int i, int j) {
    return i * w + j;
  };

  rep(j, w) rep(i, h)
  {
    if (s[i][j] == '.')
      d[f(i, j)][f(i, j)] = 0;
  }

  rep(j, w) rep(i, h)
  {
    rep(k, 4)
    {
      if (s[i][j] == '#')
        continue;

      int next_i = i + dx[k];
      int next_j = j + dy[k];
      if (0 <= next_i && next_i < h && 0 <= next_j && next_j < w)
      {
        if (s[next_i][next_j] == '.')
        {
          d[f(i, j)][f(next_i, next_j)] = 1;
          d[f(next_i, next_j)][f(i, j)] = 1;
        }
      }
    }
  }

  rep(k, h * w)
      rep(i, h * w)
          rep(j, h * w)
  {
    chmin(d[i][j], d[i][k] + d[k][j]);
  }

  int ans = -1;

  rep(b, h * w)
  {
    rep(a, h * w)
    {
      if (d[a][b] != INF)
        lout << " " << d[a][b];
      else
        lout << " X";

      if (d[a][b] != INF)
        chmax(ans, d[a][b]);
    }
    lout << endl;
  }

  cout << ans << endl;
}


Measured memory_bytes: 896