#include <bits/stdc++.h>
using namespace std;
using ll = int64_t;
using ull = uint64_t;
using uint = uint32_t;
using pii = pair<int, int>;
using pll = pair<ll, ll>;
using pil = pair<int, ll>;
using vi = vector<int>;
using vvi = vector<vi>;
using vl = vector<ll>;
template <typename T>
inline istream& operator>>(istream& i, vector<T>& v) {
  for (int j = 0; j < (v.size()); ++j) i >> v[j];
  return i;
}
template <typename T>
string join(const vector<T>& v) {
  stringstream s;
  for (int i = 0; i < (v.size()); ++i) s << ' ' << v[i];
  return s.str().substr(1);
}
template <typename T>
inline ostream& operator<<(ostream& o, const vector<T>& v) {
  if (v.size()) o << join(v);
  return o;
}
template <typename T1, typename T2>
inline istream& operator>>(istream& i, pair<T1, T2>& v) {
  return i >> v.first >> v.second;
}
template <typename T1, typename T2>
inline ostream& operator<<(ostream& o, const pair<T1, T2>& v) {
  return o << v.first << "," << v.second;
}
const double eps = 1e-6;
const ll LINF = 100200300400500600ll;
const int INF = 1001001001;
template <class T>
T chmin(T& a, T b) {
  if (a > b) {
    a = b;
    return 1;
  } else
    return 0;
}
template <class T>
T chmax(T& a, T b) {
  if (a < b) {
    a = b;
    return 1;
  } else
    return 0;
}
const int dx[4] = {0, 1, 0, -1};
const int dy[4] = {-1, 0, 1, 0};
struct D21 {
  int is, js;
  D21(int isize, int jsize) {
    is = isize;
    js = jsize;
  }
  int operator()(int i, int j) { return i * js + j; }
};
struct Tree {
  int node;
  vector<vector<ll>> dist;
  Tree() {}
  Tree(int size) {
    node = size;
    dist = vector<vector<ll>>(size, vector<ll>(size, INF));
  }
  void setDistance(int from, int to, ll cost) { dist[from][to] = cost; }
  void warshallFloyd() {
    for (int k = 0; k < node; k++)
      for (int i = 0; i < node; i++)
        for (int j = 0; j < node; j++)
          dist[i][j] = min(dist[i][j], dist[i][k] + dist[k][j]);
  }
};
void Main() {
  int h, w;
  cin >> h >> w;
  vector<string> s(h);
  cin >> s;
  D21 id(h, w);
  Tree tr(h * w);
  for (int i = 0; i < (h); ++i) {
    for (int j = 0; j < (w); ++j) {
      if (s[i][j] == '#') continue;
      int from = id(i, j);
      for (int k = 0; k < (4); ++k) {
        int ni = i + dx[k], nj = j + dy[k];
        if (ni < 0 or ni >= h or nj < 0 or nj >= w) continue;
        if (s[ni][nj] == '#') continue;
        int to = id(ni, nj);
        tr.setDistance(from, to, 1);
        tr.setDistance(to, from, 1);
      }
    }
  }
  tr.warshallFloyd();
  ll ans = 1;
  for (int i = 0; i < (h); ++i) {
    for (int j = 0; j < (w); ++j) {
      if (s[i][j] == '#') continue;
      int from = id(i, j);
      for (int i2 = 0; i2 < (h); ++i2) {
        for (int j2 = 0; j2 < (w); ++j2) {
          if (s[i2][j2] == '#') continue;
          int to = id(i2, j2);
          chmax(ans, tr.dist[from][to]);
        }
      }
    }
  }
  cout << ans << endl;
}
int main(int argc, char** argv) {
  cin.tie(nullptr);
  std::ios::sync_with_stdio(false);
  ;
  ifstream iofi;
  if (argc > 1) {
    iofi.open(argv[1]);
    std::cin.rdbuf(iofi.rdbuf());
  };
  int loop = 1;
  if (argc > 2) loop = stoi(argv[2]);
  for (int i = 0; i < (loop); ++i) Main();
}
