#include<bits/stdc++.h>
using namespace std;
#define fi first
#define se second
#define pb push_back
#define rep(i,n) for(int i = 0; i < (n); ++i)
#define rrep(i,n) for(int i = 1; i <= (n); ++i)
#define drep(i,n) for(int i = (n)-1; i >= 0; --i)
#define srep(i,s,t) for (int i = s; i < t; ++i)
#define rng(a) a.begin(),a.end()
#define rrng(a) a.rbegin(),a.rend()
#define uni(x) x.erase(unique(rng(x)),x.end())
#define PQ(T) priority_queue<T,v(T),greater<T> >
#define dup(x,y) (((x)+(y)-1)/(y))
#define v(T) vector<T>
#define vv(T) v(v(T))
#define diup(v,a,k) distance((v).begin(),(k)+upper_bound((v).begin(),(v).end(),(a)))
#define dilo(v,a,k) distance((v).begin(),(k)+lower_bound((v).begin(),(v).end(),(a)))
using ll = int64_t;
using ull = uint64_t;
using uint = uint32_t;
using pii = pair<int,int>;
using pll = pair<ll,ll>;
using pil = pair<int,ll>;
using vi = vector<int>;
using vvi = vector<vi>;
using vl = vector<ll>;
template<typename T>inline istream& operator>>(istream&i,v(T)&v)
{rep(j,v.size())i>>v[j];return i;}
template<typename T>string join(const v(T)&v)
{stringstream s;rep(i,v.size())s<<' '<<v[i];return s.str().substr(1);}
template<typename T>inline ostream& operator<<(ostream&o,const v(T)&v)
{if(v.size())o<<join(v);return o;}
template<typename T1,typename T2>inline istream& operator>>(istream&i,pair<T1,T2>&v)
{return i>>v.fi>>v.se;}
template<typename T1,typename T2>inline ostream& operator<<(ostream&o,const pair<T1,T2>&v)
{return o<<v.fi<<","<<v.se;}
const double eps = 1e-6;
const ll LINF = 100200300400500600ll;
const int INF = 1001001001;
//--------------------------------------------------------
#define fastio cin.tie(nullptr); std::ios::sync_with_stdio(false);
#define myio ifstream iofi; if(argc>1){iofi.open(argv[1]); std::cin.rdbuf(iofi.rdbuf());}
#define myloopset int loop = 1; if(argc>2) loop =stoi(argv[2]); rep(i,loop)
//--------------------------------------------------------------
template <class T> T chmin(T &a,T b){if(a>b){a=b; return 1;}else return 0;}
template <class T> T chmax(T &a,T b){if(a<b){a=b; return 1;}else return 0;}
const int dx[4] = {0,1,0,-1};
const int dy[4] = {-1,0,1,0};

struct D21{
  int is,js;
  D21(int isize,int jsize){
    is = isize;
    js = jsize;
  }
  int operator()(int i,int j){
    return i*js+j;
  }
};


//　ワーシャルフロイド法
struct Tree{
  int node;
  vector< vector<ll> > dist;
  
  Tree(){}
  Tree(int size){
    node = size;
    dist = vector <vector<ll>> (size,vector<ll>(size,LINF));
    rep(i,size) dist[i][i] = 0;
  }  

  void setDistance(int from,int to,ll cost){
    dist[from][to] = cost;
  }

  void warshallFloyd(){
    for(int k = 0;k<node;k++)
      for(int i = 0;i<node;i++)
        for(int j = 0;j<node;j++)
          dist[i][j] = min( dist[i][j], dist[i][k]+dist[k][j] );
  }
};


void Main(){
  int h,w;
  cin>>h>>w;
  v(string) s(h);
  cin>>s;
  D21 id(h,w);
  Tree tr(h*w);

  rep(i,h){
    rep(j,w){
      if(s[i][j]=='#') continue;
      int from = id(i,j);
      rep(k,4){
        int ni = i+dx[k], nj = j+dy[k];
        if(ni < 0 or ni>=h or nj<0 or nj>=w) continue;
        if(s[ni][nj] == '#') continue;
        int to = id(ni,nj);

        tr.setDistance(from,to,1);
        tr.setDistance(to,from,1);
      }
    }
  }

  tr.warshallFloyd();

  ll ans=0;
  rep(i,h){
    rep(j,w){
      if(s[i][j]=='#') continue;
      int from = id(i,j);
      rep(i2,h){
        rep(j2,w){
          if(s[i2][j2]=='#') continue;      
          int to = id(i2,j2);
          chmax(ans,tr.dist[from][to]);
        }
      }
    }
  }
  cout<<ans<<endl;

} int main(int argc,char** argv)
{fastio; myio; myloopset Main();}


Measured memory_bytes: 1536