#include <bits/stdc++.h>
#pragma GCC optimize("O3")
using namespace std;
using ll = long long;
template <class T>
using V = vector<T>;
const ll inf = (1e18);
const ll mod = 1000000007;
ll gcd(ll a, ll b) { return b ? gcd(b, a % b) : a; }
ll lcm(ll c, ll d) { return c / gcd(c, d) * d; }
struct __INIT {
  __INIT() {
    cin.tie(0);
    ios::sync_with_stdio(false);
    cout << fixed << setprecision(15);
  }
} __init;
template <class T>
bool chmax(T &a, const T &b) {
  if (a < b) {
    a = b;
    return 1;
  }
  return 0;
}
template <class T>
bool chmin(T &a, const T &b) {
  if (a > b) {
    a = b;
    return 1;
  }
  return 0;
}
V<string> s;
int h, w;
V<int> dx = {-1, 0, 1, 0}, dy = {0, -1, 0, 1};
int main() {
  cin >> h >> w;
  s.resize(h);
  for (int i = 0; i < h; i++) cin >> s[i];
  int ans = 0;
  for (int i = 0; i < h; i++) {
    for (int j = 0; j < w; j++) {
      if (s[i][j] == '#') continue;
      queue<pair<ll, ll>> q;
      q.push(pair<ll, ll>(i, j));
      V<V<int>> used(h, V<int>(w, 0));
      while (!q.empty()) {
        pair<ll, ll> now = q.front();
        q.pop();
        for (int r = 0; r < 4; r++) {
          int nx = now.first + dx[r], ny = now.second + dy[r];
          if (nx < 0 || nx >= h || ny < 0 || ny >= w) continue;
          if (s[nx][ny] == '#') continue;
          if (used[nx][ny]) continue;
          used[nx][ny] += used[now.first][now.second] + 1;
          q.push(pair<ll, ll>(nx, ny));
        }
      }
      for (int k = 0; k < h; k++)
        for (int d = 0; d < w; d++) chmax(ans, used[k][d]);
    }
  }
  cout << ans << endl;
}
