#include <bits/stdc++.h>
using namespace std;
vector<string> field;
int dx[4] = {1, 0, -1, 0};
int dy[4] = {0, 1, 0, -1};
int tmp_max = 0;
int main() {
  int h, w;
  cin >> h >> w;
  field.resize(h);
  for (int i = 0; i < h; ++i) cin >> field[i];
  for (int i = 1; i <= h; ++i) {
    for (int j = 1; j <= w; ++j) {
      int sy, sx;
      sy = i;
      sx = j;
      vector<vector<int>> dist(h, vector<int>(w, -1));
      queue<pair<int, int>> que;
      dist[sy - 1][sx - 1] = 0;
      que.push(make_pair(sx - 1, sy - 1));
      while (!que.empty()) {
        pair<int, int> v = que.front();
        que.pop();
        for (int i = 0; i < 4; ++i) {
          int nx = v.first + dx[i];
          int ny = v.second + dy[i];
          if (ny < 0 || ny >= h || nx < 0 || nx >= w) continue;
          if (field[ny][nx] == '#') continue;
          if (dist[ny][nx] != -1) continue;
          dist[ny][nx] = dist[v.second][v.first] + 1;
          que.push(make_pair(nx, ny));
        }
      }
      for (int i = 0; i < h; ++i) {
        for (int j = 0; j < w; ++j) {
          tmp_max = max(tmp_max, dist[i][j]);
        }
      }
    }
  }
  cout << tmp_max << endl;
}
