#include <bits/stdc++.h>
using namespace std;
int h, w;
int bfs(int s_h, int s_w, vector<vector<int>> &maze) {
  queue<int> q;
  vector<vector<int>> dist(h, vector<int>(w, -1));
  int p;
  p = s_h * w + s_w;
  q.push(p);
  dist.at(s_h).at(s_w) = 0;
  while (!q.empty()) {
    int front;
    front = q.front();
    int f_h = front / w;
    int f_w = front % w;
    for (int i = -1; i <= 1; i++) {
      for (int j = -1; j <= 1; j++) {
        if (0 <= f_h + i && f_h + i < h) {
          if (0 <= f_w + j && f_w + j < w) {
            if (i + j == 1 || i + j == -1) {
              if (maze.at(f_h).at(f_w) == 1) {
                if (dist.at(f_h + i).at(f_w + j) == -1) {
                  dist.at(f_h + i).at(f_w + j) = dist.at(f_h).at(f_w) + 1;
                  q.push((f_h + i) * w + f_w + j);
                }
              }
            }
          }
        }
      }
    }
    q.pop();
  }
  int ans = 0;
  for (int i = 0; i < h; i++) {
    for (int j = 0; j < w; j++) {
      ans = max(ans, dist.at(i).at(j));
    }
  }
  return ans;
}
int main() {
  cin >> h >> w;
  vector<vector<int>> maze(h, vector<int>(w));
  for (int i = 0; i < h; i++) {
    for (int j = 0; j < w; j++) {
      char c;
      cin >> c;
      if (c == '.')
        maze.at(i).at(j) = 1;
      else
        maze.at(i).at(j) = 0;
    }
  }
  int ans = 0;
  for (int i = 0; i < h; i++) {
    for (int j = 0; j < w; j++) {
      if (maze.at(i).at(j) == 1) {
        int dist;
        dist = bfs(i, j, maze);
        ans = max(ans, dist);
      }
    }
  }
  cout << ans << endl;
}
