// includes
#include <bits/stdc++.h>
using namespace std;

// macros
#define pb emplace_back
#define mk make_pair
#define FOR(i, a, b) for(int i=(a);i<(b);++i)
#define rep(i, n) FOR(i, 0, n)
#define rrep(i, n) for(int i=((int)(n)-1);i>=0;i--)
#define irep(itr, st) for(auto itr = (st).begin(); itr != (st).end(); ++itr)
#define irrep(itr, st) for(auto itr = (st).rbegin(); itr != (st).rend(); ++itr)
#define whole(x) (x).begin(),(x).end()
#define sz(x) ((int)(x).size())
#define UNIQUE(v) v.erase(unique(v.begin(), v.end()), v.end())
#define bit(n) (1LL<<(n))
// functions
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;}
//  types
using ll = long long int;
using P = pair<int, int>;
// constants
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};
// io
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;
  rep(i, h)cin >> s[i];
  GraphL g(h * w);
  rep(i, h){
    rep(j, w){
      rep(k, 4){
        if(s[i][j] == '#')continue;
        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;
  rep(i, h){
    rep(j, w){
      rep(i1, h){
        rep(j1, w){
          if(s[i][j] == '#')continue;
          if(s[i1][j1] == '#')continue;
          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;
}


Measured memory_bytes: 2176