#include <bits/stdc++.h>
using namespace std;
using ll = long long;
using ull = unsigned long long;
using pii = pair<int, int>;
int main() {
  ios::sync_with_stdio(false);
  cin.tie(0);
  int dx[4] = {1, -1, 0, 0};
  int dy[4] = {0, 0, 1, -1};
  int H, W;
  cin >> H >> W;
  vector<string> S(H);
  ll ans = 0;
  for (int i = 0; i < H; i++) cin >> S[i];
  for (int sx = 0; sx < W; sx++) {
    for (int sy = 0; sy < H; sy++) {
      if (S[sy][sx] == '#') continue;
      vector<vector<ll>> cost(H, vector<ll>(W, false));
      vector<vector<bool>> used(H, vector<bool>(W, false));
      queue<pair<pii, ll>> q;
      q.push({{sx, sy}, 0});
      while (q.size() > 0) {
        int x = q.front().first.first;
        int y = q.front().first.second;
        ll c = q.front().second;
        cost[y][x] = c;
        q.pop();
        used[y][x] = true;
        for (int i = 0; i < 4; i++) {
          int X = x + dx[i];
          int Y = y + dy[i];
          if (0 <= X && X < W && 0 <= Y && Y < H && S[Y][X] == '.' &&
              used[Y][X] == false) {
            q.push({{X, Y}, c + 1});
          }
        }
      }
      for (auto it : cost)
        for (auto it2 : it) ans = max(ans, it2);
    }
  }
  cout << ans << endl;
  return 0;
}
