#include <bits/stdc++.h>
using namespace std;
int dy4[4] = {-1, 0, +1, 0};
int dx4[4] = {0, +1, 0, -1};
int dy8[8] = {-1, -1, 0, 1, 1, 1, 0, -1};
int dx8[8] = {0, 1, 1, 1, 0, -1, -1, -1};
const long long INF = 1LL << 60;
const long long MOD = 1e9 + 7;
bool greaterSecond(const pair<int, int>& f, const pair<int, int>& s) {
  return f.second > s.second;
}
long long gcd(long long a, long long b) {
  if (b == 0) return a;
  return gcd(b, a % b);
}
long long lcm(long long a, long long b) { return a / gcd(a, b) * b; }
long long nCr(long long n, long long r) {
  if (r == 0 || r == n) {
    return 1;
  } else if (r == 1) {
    return n;
  }
  return (nCr(n - 1, r) + nCr(n - 1, r - 1));
}
long long nPr(long long n, long long r) {
  r = n - r;
  long long ret = 1;
  for (long long i = n; i >= r + 1; i--) ret *= i;
  return ret;
}
int main(void) {
  int h, w;
  cin >> h >> w;
  vector<string> s(h);
  for (int i = 0; i < h; i++) {
    cin >> s[i];
  }
  int ans = 0;
  for (int si = 0; si < h; si++) {
    for (int sj = 0; sj < w; sj++) {
      pair<int, int> start = make_pair(si, sj);
      if (s[start.first][start.second] == '#') {
        continue;
      }
      vector<vector<int>> dist(h, vector<int>(w, 0));
      vector<vector<int>> visited(h, vector<int>(w, 0));
      queue<pair<pair<int, int>, int>> q;
      q.push(make_pair(start, 0));
      int nowdist = 0;
      while (!q.empty()) {
        pair<int, int> now = q.front().first;
        int nowdist = q.front().second;
        q.pop();
        dist[now.first][now.second] = nowdist;
        visited[now.first][now.second] = 1;
        for (int i = 0; i < 4; i++) {
          pair<int, int> next =
              make_pair(now.first + dy4[i], now.second + dx4[i]);
          if (next.first < 0 || next.first >= h || next.second < 0 ||
              next.second >= w) {
            continue;
          }
          if (s[next.first][next.second] == '#') {
            continue;
          }
          if (visited[next.first][next.second] == 1) {
            continue;
          }
          q.push(make_pair(next, nowdist + 1));
        }
      }
      for (int gi = 0; gi < h; gi++) {
        for (int gj = 0; gj < w; gj++) {
          if (s[gi][gj] == '#') {
            continue;
          }
          ans = max(ans, dist[gi][gj]);
        }
      }
    }
  }
  cout << ans << endl;
}
