#include <bits/stdc++.h>
using namespace std;
template <class T>
bool chmax(T &a, const T &b) {
  if (a < b) {
    a = b;
    return 1;
  }
  return 0;
}
template <class T>
bool chmin(T &a, const T &b) {
  if (a > b) {
    a = b;
    return 1;
  }
  return 0;
}
template <typename T>
istream &operator>>(istream &is, vector<T> &vec) {
  for (auto &v : vec) is >> v;
  return is;
}
template <typename T>
ostream &operator<<(ostream &os, const vector<T> &vec) {
  for (int i = 0; i < vec.size(); i++) {
    os << vec[i];
    if (i + 1 != vec.size()) os << " ";
  }
  return os;
}
template <typename T>
ostream &operator<<(ostream &os, const set<T> &st) {
  for (auto itr = st.begin(); itr != st.end(); ++itr) {
    os << *itr;
    auto titr = itr;
    if (++titr != st.end()) os << " ";
  }
  return os;
}
template <typename T>
ostream &operator<<(ostream &os, const unordered_set<T> &st) {
  for (auto itr = st.begin(); itr != st.end(); ++itr) {
    os << *itr;
    auto titr = itr;
    if (++titr != st.end()) os << " ";
  }
  return os;
}
template <typename T>
ostream &operator<<(ostream &os, const multiset<T> &st) {
  for (auto itr = st.begin(); itr != st.end(); ++itr) {
    os << *itr;
    auto titr = itr;
    if (++titr != st.end()) os << " ";
  }
  return os;
}
template <typename T>
ostream &operator<<(ostream &os, const unordered_multiset<T> &st) {
  for (auto itr = st.begin(); itr != st.end(); ++itr) {
    os << *itr;
    auto titr = itr;
    if (++titr != st.end()) os << " ";
  }
  return os;
}
template <typename T1, typename T2>
ostream &operator<<(ostream &os, const pair<T1, T2> &p) {
  os << p.first << " " << p.second;
  return os;
}
template <typename T1, typename T2>
ostream &operator<<(ostream &os, const map<T1, T2> &mp) {
  for (auto itr = mp.begin(); itr != mp.end(); ++itr) {
    os << itr->first << ":" << itr->second;
    auto titr = itr;
    if (++titr != mp.end()) os << " ";
  }
  return os;
}
template <typename T1, typename T2>
ostream &operator<<(ostream &os, const unordered_map<T1, T2> &mp) {
  for (auto itr = mp.begin(); itr != mp.end(); ++itr) {
    os << itr->first << ":" << itr->second;
    auto titr = itr;
    if (++titr != mp.end()) os << " ";
  }
  return os;
}
using ll = long long int;
using P = pair<int, int>;
const int inf = 1e9;
const ll linf = 1LL << 50;
const double EPS = 1e-10;
const int mod = 1000000007;
const int dx[4] = {-1, 0, 1, 0};
const int dy[4] = {0, -1, 0, 1};
struct fast_io {
  fast_io() {
    ios_base::sync_with_stdio(false);
    cin.tie(0);
    cout << fixed << setprecision(20);
  }
} fast_io_;
template <typename T>
struct Graph {
  int n;
  vector<vector<T>> d;
  vector<vector<int>> path;
  Graph(int n) : n(n) {
    d = vector<vector<T>>(n, vector<T>(n, numeric_limits<T>::max() / 10));
    path = vector<vector<int>>(n, vector<int>(n, -1));
    for (int i = 0; i < n; i++) d[i][i] = 0;
  }
  void warshall_floyd() {
    for (int k = 0; k < n; k++) {
      for (int i = 0; i < n; i++) {
        for (int j = 0; j < n; j++) {
          if (d[i][j] > d[i][k] + d[k][j]) {
            d[i][j] = d[i][k] + d[k][j];
            path[i][j] = k;
          }
        }
      }
    }
  }
  void adde(int at, int to, T cost) { d[at][to] = cost; }
  vector<T> &operator[](size_t i) { return d[i]; }
};
using GraphI = Graph<int>;
using GraphL = Graph<ll>;
string s[21];
int main(int argc, char const *argv[]) {
  int h, w;
  cin >> h >> w;
  for (int i = (0); i < (h); ++i) cin >> s[i];
  GraphL g(h * w);
  for (int i = (0); i < (h); ++i) {
    for (int j = (0); j < (w); ++j) {
      for (int k = (0); k < (4); ++k) {
        int nx = i + dx[k];
        int ny = j + dy[k];
        if (nx >= 0 && nx < h && ny >= 0 && ny < w && s[nx][ny] != '#') {
          g.adde(i * w + j, nx * w + ny, 1);
        }
      }
    }
  }
  g.warshall_floyd();
  ll res = 0;
  for (int i = (0); i < (h); ++i) {
    for (int j = (0); j < (w); ++j) {
      for (int i1 = (0); i1 < (h); ++i1) {
        for (int j1 = (0); j1 < (w); ++j1) {
          if (i1 == i && j1 == j) continue;
          int id1 = i * w + j, id2 = i1 * w + j1;
          if (g.d[id1][id2] < inf) {
            res = max(res, g.d[id1][id2]);
          }
        }
      }
    }
  }
  cout << res << endl;
  return 0;
}
