#include <bits/stdc++.h>
#define rep(i,n) for(int i=0; i<(n); ++i)
using namespace std;
using ll = long long;
using P = pair<int,int>

const int INF = 1001001001;
const int di[] = {-1, 0, 1, 0};
const int dj[] = {0, -1, 0, 1};

int main() {
  int h, w; cin >> h >> w;
  vector<string> s(h);
  rep(i,h) cin >> s[i];
  int ans = 0;
  rep(si,h)rep(sj,w) {
    	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();
        rep(dir,4) {
          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);
        }
    }
    rep(i,h)rep(i,w) {
      if (dist[i][j] == INF) continue;
      ans = max(ans, dist[i][j]);
    }
  }
  cout << ans << endl;
  return 0;
}