// template version 1.15
using namespace std;
#include <bits/stdc++.h>

// varibable settings
#define int long long
const int INF=1e18;

// define basic macro {{{
#define _overload3(_1,_2,_3,name,...) name
#define _rep(i,n) repi(i,0,n)
#define repi(i,a,b) for(int i=(int)(a);i<(int)(b);++i)
#define rep(...) _overload3(__VA_ARGS__,repi,_rep,)(__VA_ARGS__)
#define _rrep(i,n) rrepi(i,0,n)
#define rrepi(i,a,b) for(int i=(int)((b)-1);i>=(int)(a);--i)
#define rrep(...) _overload3(__VA_ARGS__,rrepi,_rrep,)(__VA_ARGS__)
#define each(i,a) for (auto&& i : a)
#define all(x) (x).begin(),(x).end()
#define sz(x) ((int)(x).size())
#define pb(a) push_back(a)
#define mp(a, b) make_pair(a, b)
#define mt(a, b, c) make_tuple(a, b, c)
#define divceil(a,b) ((a)+(b)-1)/(b)
#define is_in(x, a, b) ((a)<=(x) && (x)<(b))
#define uni(x) sort(all(x));x.erase(unique(all(x)),x.end())
#define ub upper_bound
#define lb lower_bound
#define posl(A, x) (lower_bound(all(A), x)-A.begin())
#define posu(A, x) (upper_bound(all(A),x)-A.begin())
template<class T> inline void chmax(T &a, const T &b) { if((a) < (b)) (a) = (b); }
template<class T> inline void chmin(T &a, const T &b) { if((a) > (b)) (a) = (b); }

typedef long long ll;
typedef vector<int> vi;
typedef vector<vi> vvi;
typedef long double ld;
typedef pair<int,int> pii;
typedef tuple<int,int,int> iii;

template<typename T> using PQ = priority_queue<T, vector<T>, greater<T>>;
struct Fast { Fast(){ std::cin.tie(0); ios::sync_with_stdio(false); } } fast;

#if defined(PCM) || defined(LOCAL)
  #include "lib/dump.hpp"
#else
  #define dump(...) 42
  #define dump_1d(...) 42
  #define dump_2d(...) 42
#endif
//}}}


using COST = int;  // TODO: update
using POS = int;  // TODO: update
using EDGE = pair<POS, COST>;/*{{{*/
struct GRAPH{
  map<POS, vector<EDGE>> all_edges;
  vector<POS> all_nodes;

  void add_edge(POS from, POS to, COST cost){
    all_edges[from].emplace_back(mp(to, cost));
  }

  auto operator[](POS pos){
    return all_edges[pos];
  }
};
auto& operator<<(auto& stream, GRAPH G){
  stream << G.all_edges;
  return stream;
}

map<POS, COST> dijkstra(GRAPH& G, POS start){
  map<POS, COST> d;  // 最短距離
  each(node, G.all_nodes) {d[node] = INF;}

  PQ<pair<COST, POS>> q;
  q.push(mp(0LL, start));
  while (!q.empty()){
    auto cp = q.top(); q.pop();
    auto cost = cp.first;
    auto pos = cp.second;

    if (cost < d[pos]) {
      d[pos] = cost;
      for (const auto &el:G[pos]){
        auto ncost = cost + el.second;
        auto npos = el.first;
        if (ncost < d[npos]) q.push(mp(ncost, npos));
      }
    }
  }
  return d;
}/*}}}*/

// int sample(){
//   // init and build G
//
//   int n,m;cin>>n>>m;
//   rep(i, m){
//     POS from, to;
//     COST cost;
//     cin>>from>>to>>cost;
//     from--;to--;
//   }
//
//   // calc min distance by G
//   POS start=...;
//   POS goal=...;
//   auto d = dijkstra(G, start);
//   cout << d[goal] << endl;
//   return 0;
// }

signed main() {
  int h,w;cin>>h>>w;
  int n = h*w;
  vvi block(h, vi(w));
  rep(i, h){
    string s;cin>>s;
    rep(j, w){
      block[i][j] = (s[j]=='.'?0:1);
    }
  }
  dump_2d(block, h, w);

  vvi g(n);
  GRAPH G;
  rep(i, n) G.all_nodes.pb(i);  // add node

  rep(i, h)rep(j, w){
    if (block[i][j]) continue;
    int u = i*w+j;

    vi dd = {1, w}; 
    rep(k, sz(dd)){
      int v = u+dd[k];
      if (0<=v && v<h*w){
        if (block[v/w][v%w]) continue;
        if (v%w==0 && dd[k]==1) continue;
        g[u].pb(v);
        g[v].pb(u);
        G.add_edge(u, v, 1);
        G.add_edge(v, u, 1);
      }
    }
  }
  dump(g);
  
  int ans = 1;
  rep(i, h)rep(j, w){
    if (block[i][j]) continue;

    int u = i*w+j;
    // auto dist = dijkstra(G, u);
    // each(el, dist) if (el.second!=INF) chmax(ans, el.second);
    // continue;

    queue<int> q;
    vector<int> used(n);
    q.push(u);
    vi d(n, INF);
    d[u] = 0;
    while (!q.empty()){
      auto u = q.front();q.pop();
      if (used[u]) continue;
      used[u] = 1;

      each(v, g[u]){
        if (d[u]+1<d[v]){
          d[v] = d[u]+1;
          q.push(v);
        }
      }
    }
    rep(i, n) if (d[i]!=INF) chmax(ans, d[i]);
  }
  cout << ans << endl;

  return 0;
}
