#include <bits/stdc++.h>
using namespace std;
using ll = long long;
using P = pair<int, int>;
const int INF = 10000;
const int di[] = {-1, 0, 1, 0};
const int dj[] = {0, -1, 0, 1};
int main() {
  int w, h, ans = 0;
  char c;
  cin >> w >> h;
  vector<string> s(h);
  for (int i = 0; i < h; i++) cin >> s[i];
  for (int si = 0; si < h; si++)
    for (int sj = 0; sj < w; sj++) {
      if (s[si][sj] == '#') continue;
      vector<vector<int>> dist(h, vector<int>(w, INF));
      queue<P> q;
      auto update = [&](int i, int j, int x) {
        if (dist[i][j] != INF) return;
        dist[i][j] = x;
        q.push(P(i, j));
      };
      update(si, sj, 0);
      while (!q.empty()) {
        int i = q.front().first;
        int j = q.front().second;
        q.pop();
        for (int dir = 0; dir < 4; dir++) {
          int ni = i + di[dir], nj = j + dj[dir];
          if (ni < 0 || ni >= h || nj < 0 || nj >= w) continue;
          if (s[ni][nj] == '#') continue;
          update(ni, nj, dist[i][j] + 1);
        }
      }
      for (int i = 0; i < h; i++)
        for (int j = 0; j < w; j++) {
          if (dist[i][j] == INF) continue;
          ans = max(ans, dist[i][j]);
        }
    }
  cout << ans << endl;
}
