#include <bits/stdc++.h>
using namespace std;
using ll = long long int;
using ull = unsigned long long int;
using ld = long double;
using pii = pair<int, int>;
using pll = pair<ll, ll>;
int dx[] = {1, 0, -1, 0};
int dy[] = {0, 1, 0, -1};
const int INT_INF = (int)(2e9);
const ll LL_INF = (ll)(2e18);
const int NIL = -1;
static mt19937 _g(time(nullptr));
std::string pad(int num) {
  char buffer[4];
  std::snprintf(buffer, sizeof(buffer), "%03d", num);
  return buffer;
}
inline ll randint(ll a, ll b) {
  ll w = (_g() << 31LL) ^ _g();
  return a + w % (b - a + 1);
}
inline void fast_io() {
  ios_base::sync_with_stdio(false);
  cin.tie(nullptr);
  cout.tie(nullptr);
};
template <typename T>
inline T sign(T x) {
  return T(x > 0) - T(x < 0);
}
template <typename T, typename S>
inline ostream& operator<<(ostream& os, const pair<T, S> p) {
  cout << "[" << p.first << ";" << p.second << "]";
  return os;
}
template <typename T>
inline ostream& operator<<(ostream& os, const vector<T>& v) {
  for (auto el : v) cout << el << " ";
  return os;
}
template <typename T>
inline T fetch() {
  T ret;
  cin >> ret;
  return ret;
}
template <typename T>
inline vector<T> fetch_vec(int sz) {
  vector<T> ret(sz);
  for (auto& elem : ret) cin >> elem;
  return ret;
}
int H, W;
int ans = 0;
string Maze[30];
int dist(int const& i, int const& j, int const& k, int const& l) {
  if (i == k and j == l) return 0;
  if (Maze[i][j] == '#') return 0;
  if (Maze[k][l] == '#') return 0;
  bool visited[30][30];
  visited[i][j] = true;
  stack<pii> container;
  map<pii, pii> toGoalPath;
  container.push(make_pair(i, j));
  while (!container.empty()) {
    pii now = container.top();
    container.pop();
    for (int k = 0; k < 4; k++) {
      int x, y;
      x = now.first + dx[k];
      y = now.second + dy[k];
      if (x < 0 | y < 0 | x >= H | y >= W) continue;
      if (visited[x][y])
        continue;
      else
        visited[x][y] = true;
      if (x == k and y == l) {
        toGoalPath.emplace(make_pair(x, y), now);
        break;
      }
      if ((Maze[x][y] == '#')) continue;
      if ((Maze[x][y] == '.')) {
        container.push(make_pair(x, y));
        toGoalPath.emplace(make_pair(x, y), now);
      }
    }
  }
  int ans = 0;
  pii P = make_pair(k, l);
  while (P != make_pair(i, j)) {
    P = toGoalPath[P];
    ans++;
  }
  return ans;
}
int main() {
  fast_io();
  cin >> H >> W;
  for (int i = 0; i < H; i++) {
    cin >> Maze[i];
  }
  for (int i = 0; i < H; i++)
    for (int j = 0; j < W; j++)
      for (int k = 0; k < H; k++)
        for (int l = 0; l < W; l++) ans = max(ans, dist(i, j, k, l));
  ;
  cout << ans << endl;
  return 0;
}
