#include <bits/stdc++.h>
using namespace std;
void ans(bool b) { cout << (b ? "Yes" : "No") << endl; }
void ans2(bool b) { cout << (b ? "YES" : "NO") << endl; }
template <typename T>
inline bool chmax(T& a, T b) {
  if (a < b) a = b;
  return a < b;
}
template <typename T>
inline bool chmin(T& a, T b) {
  if (b < a) a = b;
  return b < a;
}
template <typename A, size_t N, typename T>
void Fill(A (&array)[N], const T& val) {
  fill((T*)array, (T*)(array + N), val);
}
template <typename T>
void vdeb(T v) {
  cout << "#vector set debug" << endl;
  for (auto vv : v) cout << vv << " ";
  cout << endl;
}
template <typename T1, typename T2>
void adeb(T1 arr[], T2 n) {
  cout << "#adebug" << endl;
  for (int i = 0; i <= n; i++) cout << arr[i] << " ";
  cout << endl;
}
template <typename T1>
void mdeb(T1 mp) {
  cout << "#map pair debug" << endl;
  for (auto const& m : mp) cout << m.first << " " << m.second << endl;
}
int dx[] = {1, 0, -1, 0, 1, -1, 1, -1};
int dy[] = {0, 1, 0, -1, 1, -1, -1, 1};
class DMazeMaster {
 public:
  void solve(std::istream& in, std::ostream& out) {
    int h, w;
    in >> h >> w;
    char s[100][100];
    for (long long i = 0; i < h; i++) {
      for (long long j = 0; j < w; j++) {
        in >> s[i][j];
      }
    }
    int res = 0;
    for (long long si = 0; si < h; si++) {
      for (long long sj = 0; sj < w; sj++) {
        if (s[si][sj] == '#') continue;
        for (long long gi = 0; gi < h; gi++) {
          for (long long gj = 0; gj < w; gj++) {
            if (s[gi][sj] == '#') continue;
            queue<pair<int, int>> q;
            q.push(make_pair(si, sj));
            vector<vector<int>> dist(h, vector<int>(w, 0));
            dist[si][sj] = 0;
            while (!q.empty()) {
              pair<int, int> u = q.front();
              q.pop();
              for (long long i = 0; i < 4; i++) {
                int dly = u.first + dy[i];
                int dlx = u.second + dx[i];
                if (dlx < 0 || dlx >= w || dly < 0 || dly >= h) continue;
                if (dist[dly][dlx] || (dly == si && dlx == sj)) continue;
                if (s[dly][dlx] == '#') continue;
                dist[dly][dlx] = dist[u.first][u.second] + 1;
                q.push(make_pair(dly, dlx));
              }
            }
            chmax(res, dist[gi][gj]);
          }
        }
      }
    }
    out << res << endl;
  }
};
int main() {
  DMazeMaster solver;
  std::istream& in(std::cin);
  std::ostream& out(std::cout);
  solver.solve(in, out);
  return 0;
}
