#include <bits/stdc++.h>
using namespace std;
template <typename A, typename B>
string to_string(pair<A, B> p);
template <typename A, typename B, typename C>
string to_string(tuple<A, B, C> p);
template <typename A, typename B, typename C, typename D>
string to_string(tuple<A, B, C, D> p);
string to_string(const string &s) { return '"' + s + '"'; }
string to_string(const char *s) { return to_string((string)s); }
string to_string(bool b) { return (b ? "true" : "false"); }
string to_string(vector<bool> v) {
  bool first = true;
  string res = "{";
  for (int i = 0; i < static_cast<int>(v.size()); i++) {
    if (!first) {
      res += ", ";
    }
    first = false;
    res += to_string(v[i]);
  }
  res += "}";
  return res;
}
template <size_t N>
string to_string(bitset<N> v) {
  string res = "";
  for (size_t i = 0; i < N; i++) {
    res += static_cast<char>('0' + v[i]);
  }
  return res;
}
template <typename A>
string to_string(A v) {
  bool first = true;
  string res = "{";
  for (const auto &x : v) {
    if (!first) {
      res += ", ";
    }
    first = false;
    res += to_string(x);
  }
  res += "}";
  return res;
}
template <typename A, typename B>
string to_string(pair<A, B> p) {
  return "(" + to_string(p.first) + ", " + to_string(p.second) + ")";
}
template <typename A, typename B, typename C>
string to_string(tuple<A, B, C> p) {
  return "(" + to_string(get<0>(p)) + ", " + to_string(get<1>(p)) + ", " +
         to_string(get<2>(p)) + ")";
}
template <typename A, typename B, typename C, typename D>
string to_string(tuple<A, B, C, D> p) {
  return "(" + to_string(get<0>(p)) + ", " + to_string(get<1>(p)) + ", " +
         to_string(get<2>(p)) + ", " + to_string(get<3>(p)) + ")";
}
void debug_out() { cerr << endl; }
template <typename Head, typename... Tail>
void debug_out(Head H, Tail... T) {
  cerr << " " << to_string(H);
  debug_out(T...);
}
template <class T = int>
using V = vector<T>;
template <class T = int>
using VV = V<V<T>>;
int r, c;
int dx[4] = {0, 0, 1, -1};
int dy[4] = {1, -1, 0, 0};
map<pair<pair<int, int>, pair<int, int>>, int> mm;
int vis[25][25];
int findlen(vector<string> &vs, int a, int b, int aa, int bb) {
  if (mm[{{a, b}, {aa, bb}}]) return mm[{{a, b}, {aa, bb}}];
  queue<pair<pair<int, int>, int>> q;
  memset(vis, 0, sizeof vis);
  q.push({{a, b}, 0});
  while (q.size()) {
    pair<pair<int, int>, int> t = q.front();
    q.pop();
    if (t.first == make_pair(aa, bb)) {
      mm[{{a, b}, {aa, bb}}] = t.second;
      mm[{{aa, bb}, {a, b}}] = t.second;
      return t.second;
    }
    int x = t.first.first, y = t.first.second;
    vis[x][y] = 1;
    for (int i = 0; i < 4; i++) {
      if (x + dx[i] < 0 || x + dx[i] >= vs.size() || y + dy[i] < 0 ||
          y + dy[i] >= vs[0].size() || vs[x + dx[i]][y + dy[i]] == '#' ||
          vis[dx[i] + x][dy[i] + y])
        continue;
      q.push({{x + dx[i], y + dy[i]}, t.second + 1});
    }
  }
}
int main() {
  cin.tie(nullptr);
  ios::sync_with_stdio(false);
  cin >> r >> c;
  vector<string> vs(r);
  for (int i = 0; i < r; i++) {
    cin >> vs[i];
  }
  int res = 0;
  for (int i = 0; i < r; i++) {
    for (int j = 0; j < c; j++) {
      if (vs[i][j] == '.') {
        for (int ii = 0; ii < r; ii++) {
          for (int jj = 0; jj < c; jj++) {
            if (vs[ii][jj] == '.' && (ii != i || jj != j)) {
              res = max(res, findlen(vs, i, j, ii, jj));
            }
          }
        }
      }
    }
  }
  cout << res << endl;
}
