#include <bits/stdc++.h>
using namespace std;
const int mod = 1000000007;
int bfs(int c, int r);
vector<vector<int> > M(20, vector<int>(20, 0));
int H, W;
int dx[4] = {0, 0, 1, -1};
int dy[4] = {1, -1, 0, 0};
int main() {
  cin >> H >> W;
  vector<pair<int, int> > P;
  for (int i = 0; i < H; i++) {
    string S;
    cin >> S;
    for (int j = 0; j < W; j++) {
      if (S[j] == '.') {
        M[i][j] = 1;
        P.push_back(make_pair(i, j));
      }
    }
  }
  int ans = 0;
  for (int i = 0; i < P.size(); i++) {
    int dist = bfs(P[i].first, P[i].second);
    ans = max(dist, ans);
  }
  cout << ans << '\n';
}
int bfs(int c, int r) {
  vector<vector<int> > visited(20, vector<int>(20, -1));
  queue<pair<int, int> > q;
  q.push(make_pair(c, r));
  visited[c][r] = 0;
  int next_c, next_r;
  int max_dist = 0;
  pair<int, int> point;
  while (!q.empty()) {
    point = q.front();
    q.pop();
    for (int i = 0; i < 4; i++) {
      next_c = point.first + dx[i];
      next_r = point.second + dy[i];
      if (next_c < 0 || H < next_c || next_r < 0 || W < next_r) {
        continue;
      }
      if (visited[next_c][next_r] != -1) continue;
      if (M[next_c][next_r] == 0) continue;
      visited[next_c][next_r] = visited[point.first][point.second] + 1;
      q.push(make_pair(next_c, next_r));
      max_dist = max(visited[next_c][next_r], max_dist);
    }
  }
  for (int i = 0; i < H; i++) {
    for (int j = 0; j < W; j++) {
      max_dist = max(visited[i][j], max_dist);
    }
  }
  return max_dist;
}
