#include <bits/stdc++.h>
using namespace std;
int main() {
  int h, w;
  cin >> h >> w;
  vector<string> grid(h);
  for (int i = 0; i < h; i++) {
    cin >> grid[i];
  }
  int ans = 0;
  for (int i = 0; i < w; i++) {
    for (int j = 0; j < h; j++) {
      int sx = i;
      int sy = j;
      for (int k = 0; k < w; k++) {
        for (int l = 0; l < h; l++) {
          int gx = k;
          int gy = l;
          if (grid[sy][sx] == '.' && grid[gy][gx] == '.') {
            queue<pair<int, pair<int, int>>> que;
            vector<vector<int>> dist(h, vector<int>(w, -1));
            que.push(make_pair(0, make_pair(sy, sx)));
            dist[sy][sx] = 0;
            while (que.size()) {
              pair<int, pair<int, int>> now = que.front();
              que.pop();
              int d = now.first;
              int now_y = now.second.first;
              int now_x = now.second.second;
              if (now_x == gx && now_y == gy) {
                ans = max(ans, d);
                break;
              }
              int dy[4] = {0, 1, 0, -1};
              int dx[4] = {1, 0, -1, 0};
              for (int i = 0; i < 4; i++) {
                int ny = now_y + dy[i];
                int nx = now_x + dx[i];
                if (0 <= nx && nx < w && 0 <= ny && ny < h &&
                    dist[ny][nx] == -1 && grid[ny][nx] == '.') {
                  que.push(make_pair(d + 1, make_pair(ny, nx)));
                }
              }
            }
          }
        }
      }
    }
  }
  cout << ans << endl;
}
