#include <bits/stdc++.h>
#define pb push_back
#define eb emplace_back
#define fi first
#define se second
#define rep(i,N) for(long long i = 0; i < (long long)(N); i++)
#define repr(i,N) for(long long i = (long long)(N) - 1; i >= 0; i--)
#define rep1(i,N) for(long long i = 1; i <= (long long)(N) ; i++)
#define repr1(i,N) for(long long i = (N) ; (long long)(i) > 0 ; i--)
#define each(x,v) for(auto& x : v)
#define all(v) (v).begin(),(v).end()
#define sz(v) ((int)(v).size())
#define ini(...) int __VA_ARGS__; in(__VA_ARGS__)
#define inl(...) long long __VA_ARGS__; in(__VA_ARGS__)
#define ins(...) string __VA_ARGS__; in(__VA_ARGS__)
using namespace std; void solve();
using ll = long long; template<class T = ll> using V = vector<T>;
using vi = V<int>; using vl = V<>; using vvi = V< V<int> >;
constexpr int inf = 1001001001; constexpr ll infLL = (1LL << 61) - 1;
struct IoSetupNya {IoSetupNya() { cin.tie(nullptr); ios::sync_with_stdio(false); cout << fixed << setprecision(15); cerr << fixed << setprecision(7);} } iosetupnya;
template<typename T, typename U> inline bool amin(T &x, U y) { return (y < x) ? (x = y, true) : false; }
template<typename T, typename U> inline bool amax(T &x, U y) { return (x < y) ? (x = y, true) : false; }
template<typename T, typename U> ostream& operator <<(ostream& os, const pair<T, U> &p) { os << p.first << " " << p.second; return os; }
template<typename T, typename U> istream& operator >>(istream& is, pair<T, U> &p) { is >> p.first >> p.second; return is; }
template<typename T> ostream& operator <<(ostream& os, const vector<T> &v) { int s = (int)v.size(); rep(i,s) os << (i ? " " : "") << v[i]; return os; }
template<typename T> istream& operator >>(istream& is, vector<T> &v) { for(auto &x : v) is >> x; return is; }
void in(){} template <typename T,class... U> void in(T &t,U &...u){ cin >> t; in(u...);}
void out(){cout << "\n";} template <typename T,class... U> void out(const T &t,const U &...u){ cout << t; if(sizeof...(u)) cout << " "; out(u...);}
template<typename T>void die(T x){out(x); exit(0);}
#ifdef NyaanDebug
  #include "NyaanDebug.h"
  #define trc(...) do { cerr << #__VA_ARGS__ << " = "; dbg_out(__VA_ARGS__);} while(0)
  #define trca(v,N) do { cerr << #v << " = "; array_out(v , N);cout << endl;} while(0)
#else
  #define trc(...)
  #define trca(...)
  int main(){solve();}
#endif

using P = pair<int,int>; using vp = V<P>;
constexpr int MOD = /**/ 1000000007; //*/ 998244353;
//////////

//グリッド
#ifdef NyaanDebug
  #define gtrc() g_trace()
#else 
  #define gtrc() 1
#endif
int H,W; vector<string> g;
const int dx[]={1,0,-1,0}; const int dy[]={0,1,0,-1};const int dn = 4;
//const int dx[]={1,1,1,0,-1,-1,-1,0}; const int dy[]={1,0,-1,-1,-1,0,1,1};const int dn = 8;
//using PP = pair<pair<int,int>,pair<int,int>>;
inline bool ok(int x,int y){return 0<=x && x<H && 0<=y && y<W ;}
void g_trace(){ rep(i,H) cerr << g[i] << endl; }
void grid(){ cin>>H>>W; g.resize(H); rep(i,H) cin>>g[i]; }

// 依存ライブラリ _grid
// grid上を幅優先探索して距離を調べる
void gbfs(vvi &d , pair<int,int> start){
  using PP = pair<pair<int,int> , int>;
  d.resize( H , vi(W , -1) );
  queue<PP> Q;
  Q.emplace( start , 0 );
  while(!Q.empty()){
    PP cur = Q.front(); Q.pop();
    int x = cur.first.first , y = cur.first.second , dep = cur.second;
    if(d[x][y] != -1) continue;
    d[x][y] = dep;
    rep(i , dn){
      int dh = x + dx[i] , dw = y + dy[i];
      if( ok(dh , dw) && d[dh][dw] == -1){
        if( g[dh][dw] == '.' ){ // ここに条件を書く
          Q.emplace( make_pair(dh , dw) , dep + 1 );
        }
      }
    }
  }
}



void solve(){
  grid();
  trc(g);
  ll ans = 0;
  rep(i , H) rep(j , W){
    vvi d;
    if(g[i][j] == '#') continue;
    gbfs(d , P(i , j));
    trc(d);
    rep(k , H) rep(l , W) amax(ans , d[k][l]);
  }
  out(ans);
}

Measured memory_bytes: 384