#include <bits/stdc++.h>
using namespace std;
int main() {
  int H, W;
  cin >> H >> W;
  vector<string> S(H);
  for (int i = 0; i < H; ++i) {
    cin >> S[i];
  }
  int sx = H - 1;
  int sy = 0;
  const int dx[4] = {1, 0, -1, 0};
  const int dy[4] = {0, 1, 0, -1};
  int ans = -1;
  for (int sxi = 0; sxi < W; ++sxi) {
    for (int syi = 0; syi < H; ++syi) {
      sx = sxi;
      sy = syi;
      if (S[sx][sy] == '#') continue;
      int maxdist = -1;
      vector<vector<int>> dist(H, vector<int>(W, -1));
      dist[sx][sy] = 0;
      vector<vector<int>> prev_x(H, vector<int>(W, -1));
      vector<vector<int>> prev_y(H, vector<int>(W, -1));
      queue<pair<int, int>> que;
      que.push(make_pair(sx, sy));
      while (!que.empty()) {
        pair<int, int> cur = que.front();
        int x = cur.first;
        int y = cur.second;
        que.pop();
        for (int dir = 0; dir < 4; ++dir) {
          int next_x = x + dx[dir];
          int next_y = y + dy[dir];
          if (next_x < 0 || next_x >= H || next_y < 0 || next_y >= W) continue;
          if (S[next_x][next_y] == '#') continue;
          if (dist[next_x][next_y] == -1) {
            que.push(make_pair(next_x, next_y));
            dist[next_x][next_y] = dist[x][y] + 1;
            prev_x[next_x][next_y] = x;
            prev_y[next_x][next_y] = y;
          }
        }
      }
      for (int i = 0; i < H; ++i) {
        for (int j = 0; j < W; ++j) {
          maxdist = max(maxdist, dist[i][j]);
        }
      }
      ans = max(ans, maxdist);
    }
  }
  cout << ans << endl;
  return 0;
}
