#include <bits/stdc++.h>
#ifdef LOCAL_DEBUG
#define DEBUG 1
#define CERR if(DEBUG) cerr
#define MACRO(_1,_2,_3,_4,_5,_6,_7,_8,_9,_10,NAME,...) NAME
#define pr(...) CERR<<MACRO(__VA_ARGS__,pr10,pr9,pr8,pr7,pr6,pr5,pr4,pr3,pr2,pr1)(__VA_ARGS__)<<endl
#define pr1(a) (#a)<<"="<<(a)<<" "
#define pr2(a,b) pr1(a)<<pr1(b)
#define pr3(a,b,c) pr1(a)<<pr2(b,c)
#define pr4(a,b,c,d) pr1(a)<<pr3(b,c,d)
#define pr5(a,b,c,d,e) pr1(a)<<pr4(b,c,d,e)
#define pr6(a,b,c,d,e,f) pr1(a)<<pr5(b,c,d,e,f)
#define pr7(a,b,c,d,e,f,g) pr1(a)<<pr6(b,c,d,e,f,g)
#define pr8(a,b,c,d,e,f,g,h) pr1(a)<<pr7(b,c,d,e,f,g,h)
#define pr9(a,b,c,d,e,f,g,h,i) pr1(a)<<pr8(b,c,d,e,f,g,h,i)
#define pr10(a,b,c,d,e,f,g,h,i,j) pr1(a)<<pr9(b,c,d,e,f,g,h,i,j)
#define prArr(a){CERR<<(#a)<<"={";int i=0;for(auto t:(a))CERR<<(i++?", ":"")<<t;CERR<<"}"<<endl;}
#else
#define DEBUG 0
#define pr(...)
#define prArr(a)
#endif
using namespace std;
using Int = long long;
using _int = int;
using ll = long long;
using Double = long double;
const Int INF = (1LL<<60)+1e9; // ~ 1.15 * 1e18
const Int mod = (1e9)+7;
const Double EPS = 1e-8;
const Double PI = 6.0 * asin((Double)0.5);
using P = pair<Int,Int>;
template<class T> T Max(T &a,T b){return a=max(a,b);}
template<class T> T Min(T &a,T b){return a=min(a,b);}
template<class T1, class T2> ostream& operator<<(ostream& o,pair<T1,T2> p){return o<<"("<<p.first<<","<<p.second<<")";}
template<class T1, class T2, class T3> ostream& operator<<(ostream& o,tuple<T1,T2,T3> t){return o<<"("<<get<0>(t)<<","<<get<1>(t)<<","<<get<2>(t)<<")";}
template<class T1, class T2> istream& operator>>(istream& i,pair<T1,T2> &p){return i>>p.first>>p.second;}
template<class T> ostream& operator<<(ostream& o,vector<T> a){Int i=0;for(T t:a)o<<(i++?" ":"")<<t;return o;}
template<class T> istream& operator>>(istream& i,vector<T> &a){for(T &t:a)i>>t;return i;}
//INSERT ABOVE HERE

template<typename ctype, ctype INF>
class Dijkstra{
public:
  typedef tuple<Int,ctype> T;  //for edge
  typedef tuple<ctype,Int> T2; //for priority_queue
  Int V;
  vector<vector<T> > G;
  vector<vector<T> > rG;

  Dijkstra():V(-1){}
  Dijkstra(Int V):V(V),G(V),rG(V){}

  void add_edge(Int a,Int b,ctype c,Int oneway = 0){
    assert(a >= 0 && b >= 0);
    assert(a < V && b < V);
    G[a].push_back(T(b, c));
    rG[b].push_back(T(a, c));
    if(!oneway) G[b].push_back(T(a, c)), rG[a].push_back(T(b, c));
  }

  vector<ctype> dijkstra(Int start, Int rev = 0){
    vector<ctype> D(V, INF);
    vector<Int> visited(V,0);
    priority_queue<T2, vector<T2>, greater<T2> > Q;
    Q.push(T2(0, start));
    D[start] = 0;
    while(!Q.empty()){
      ctype cost;
      Int pos;
      tie(cost,pos) = Q.top(); Q.pop();
      assert(!visited[pos] || D[pos] <= cost);
      if(visited[pos]++) continue;

      for(auto t:rev == 0? G[pos]:rG[pos]){
	Int to = get<0>(t);
        ctype ncost = cost + get<1>(t);
        if(D[to] <= ncost) continue;
        D[to] = ncost;
        Q.push(T2(ncost, to));
      }
    }
    return D;
  }

  vector<vector<Int> > getDAG(Int start, Int goal){
    auto DS = dijkstra(start, 0);
    auto DT = dijkstra(goal, 1);
    vector<vector<Int> > DAG(V);
    vector<Int> visited(V,0);
    queue<Int> Q;
    Q.push(start);
    while(!Q.empty()){
      Int pos; pos = Q.front(); Q.pop();
      if(visited[pos]++) continue;

      for(auto t:G[pos]){
        Int to = get<0>(t);
        ctype cost = get<1>(t);
	if(DS[pos] + DT[to] + cost != DS[goal]) continue;
	Q.push(to);
	DAG[pos].push_back(to);
      }
    }
    return DAG;
  }
};

signed main(){
  srand((unsigned)time(NULL));
  cin.tie(0);
  ios_base::sync_with_stdio(0);
  cout << fixed << setprecision(12);


  Int H, W;
  cin>>H>>W;
  vector<string> mp(H);
  cin>>mp;


  Int dx[] = {0, 0, 1, -1};
  Int dy[] = {1, -1, 0, 0};

  Dijkstra <Int, INF> dij(H * W);
  for(Int i=0;i<H;i++)
    for(Int j=0;j<W;j++){
      for(Int k=0;k<4;k++){
        Int ni = i + dy[k];
        Int nj = j + dx[k];
        if(ni < 0 || ni >= H || nj < 0 || nj >= W || mp[i][j] == '#' || mp[ni][nj] == '#') continue;
        Int a = i * W + j;
        Int b = ni * W + nj;
        dij.add_edge(a, b, 1, 1);
      }
    }


  Int ans = 0;
  for(Int i=0;i<H;i++)
    for(Int j=0;j<W;j++){
      if(mp[i][j] == '#') continue;
      Int S = i * W + j;
      auto D = dij.dijkstra(S);
      for(Int s=0;s<H;s++)
        for(Int t=0;t<W;t++){
          if(mp[s][t] == '#') continue;
          Int T = s * W + t;
          Max(ans, D[T]);
        }
    }
  cout<<ans<<endl;
  return 0;
}


Measured memory_bytes: 384