#include <bits/stdc++.h>
using namespace std;
//#define int long long
//TEMPLATE START---------------8<---------------8<---------------8<---------------8<---------------//
typedef long long ll;       typedef long double ld;  typedef pair<int,int> pii; typedef pair<ll,ll> pll;  typedef vector<int> vi;   typedef vector<ll> vl;
typedef vector<string> vst; typedef vector<bool> vb; typedef vector<ld> vld;    typedef vector<pii> vpii; typedef vector<pll> vpll; typedef vector<vector<int> > vvi;
const int INF = (0x7FFFFFFFL); const ll INFF = (0x7FFFFFFFFFFFFFFFL); const string ALPHABET = "ABCDEFGHIJKLMNOPQRSTUVWXYZ";
const int MOD = 1e9 + 7;       const int MODD = 998244353;            const string alphabet = "abcdefghijklmnopqrstuvwxyz";
const double PI = acos(-1.0);  const double EPS = 1e-9;               const string Alphabet = "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz";
int dx[9] = { 1, 0, -1,  0,  1, -1, -1, 1, 0 };
int dy[9] = { 0, 1,  0, -1, -1, -1,  1, 1, 0 };
#define ln '\n'
#define scnaf scanf
#define sacnf scanf
#define sancf scanf
#define SS(type, ...)type __VA_ARGS__;MACRO_VAR_Scan(__VA_ARGS__);
template<typename T> void MACRO_VAR_Scan(T& t){cin >> t;}template<typename First, typename...Rest> void MACRO_VAR_Scan(First& first, Rest&...rest){cin >> first;MACRO_VAR_Scan(rest...);}
#define SV(type,c,n) vector<type> c(n);for(auto& i:c)cin >> i;
#define SVV(type,c,n,m) vector<vector<type>> c(n,vector<type>(m));for(auto& r:c)for(auto& i:r)cin >> i;
template<class T,class U>ostream &operator<<(ostream &o,const pair<T,U>&j){o<<"{"<<j.first<<", "<<j.second<<"}";return o;}
template<class T,class U>ostream &operator<<(ostream &o,const map<T,U>&j){o<<"{";for(auto t=j.begin();t!=j.end();++t)o<<(t!=j.begin()?", ":"")<<*t;o<<"}";return o;}
template<class T>ostream &operator<<(ostream &o,const set<T>&j){o<<"{";for(auto t=j.begin();t!=j.end();++t)o<<(t!=j.begin()?", ":"")<<*t;o<<"}";return o;}
template<class T>ostream &operator<<(ostream &o,const multiset<T>&j){o<<"{";for(auto t=j.begin();t!=j.end();++t)o<<(t!=j.begin()?", ":"")<<*t;o<<"}";return o;}
template<class T>ostream &operator<<(ostream &o,const vector<T>&j){o<<"{";for(int i=0;i<(int)j.size();++i)o<<(i>0?", ":"")<<j[i];o<<"}";return o;}
inline int print(void){cout << endl; return 0;}
template<class Head> int print(Head&& head){cout << head;print();return 0;} template<class Head,class... Tail> int print(Head&& head,Tail&&... tail){cout<<head<<" ";print(forward<Tail>(tail)...);return 0;}
#ifdef LOCAL
inline int dump(void){cerr << endl; return 0;}
template<class Head> int dump(Head&& head){cerr << head;dump();return 0;}
template<class Head,class... Tail> int dump(Head&& head,Tail&&... tail){cerr<<head<<" ";dump(forward<Tail>(tail)...);return 0;}
#define debug(...) do{cerr<<__LINE__<<":	"<<#__VA_ARGS__<<" = ";dump(__VA_ARGS__);}while(0)
#define ER(x)  cerr << #x << " = " << (x) << endl;
#define ERV(v) {cerr << #v << " : ";for(const auto& xxx : v){cerr << xxx << " ";}cerr << "\n";}
#else
#define dump(...)
#define debug(...)
#define ER(x)
#define ERV(v)
#endif
template<typename T> void PA(T &a){int ASIZE=sizeof(a)/sizeof(a[0]);for(int ii=0;ii<ASIZE;++ii){cout<<a[ii]<<" \n"[ii==ASIZE-1];}}
template<typename T> void PV(T &v){int VSIZE=v.size();for(int ii=0;ii<VSIZE;++ii){cout<<v[ii]<<" \n"[ii==VSIZE-1];}}
inline int YES(bool x){cout<<((x)?"YES":"NO")<<endl;return 0;} inline int Yes(bool x){cout<<((x)?"Yes":"No")<<endl;return 0;}  inline int yes(bool x){cout<<((x)?"yes":"no")<<endl;return 0;}
inline int yES(bool x){cout<<((x)?"yES":"nO")<<endl;return 0;} inline int Yay(bool x){cout<<((x)?"Yay!":":(")<<endl;return 0;}
template<typename A,typename B> void sankou(bool x,A a,B b){cout<<((x)?(a):(b))<<endl;}
#define _overload3(_1,_2,_3,name,...) name
#define _REP(i,n) REPI(i,0,n)
#define REPI(i,a,b) for(ll i=ll(a);i<ll(b);++i)
#define REP(...) _overload3(__VA_ARGS__,REPI,_REP,)(__VA_ARGS__)
#define _RREP(i,n) RREPI(i,n,0)
#define RREPI(i,a,b) for(ll i=ll(a);i>=ll(b);--i)
#define RREP(...) _overload3(__VA_ARGS__,RREPI,_RREP,)(__VA_ARGS__)
#define EACH(e,v) for(auto& e : v)
#define PERM(v) sort((v).begin(),(v).end());for(bool c##p=1;c##p;c##p=next_permutation((v).begin(),(v).end()))
#define ADD(a,b) a=(a+ll(b))%MOD
#define MUL(a,b) a=(a*ll(b))%MOD
inline ll MOP(ll x,ll n,ll m=MOD){ll r=1;while(n>0){if(n&1)(r*=x)%=m;(x*=x)%=m;n>>=1;}return r;}
inline ll gcd(ll a,ll b){return b?gcd(b,a%b):a;}inline ll lcm(ll a,ll b){return a*b/gcd(a,b);}inline ll POW(ll a,ll b){ll c=1ll;do{if(b&1)c*=1ll*a;a*=1ll*a;}while(b>>=1);return c;}
template<typename T,typename A,typename B> inline bool between(T x,A a,B b) {return ((a<=x)&&(x<b));}template<class T> inline T sqr(T x){return x*x;}
template<typename A,typename B> inline bool chmax(A &a,const B &b){if(a<b){a=b;return 1;}return 0;}
template<typename A,typename B> inline bool chmin(A &a,const B &b){if(a>b){a=b;return 1;}return 0;}
#define tmax(x,y,z) max((x),max((y),(z)))
#define tmin(x,y,z) min((x),min((y),(z)))
#define PB emplace_back
#define MP make_pair
#define MT make_tuple
#define all(v) (v).begin(),(v).end()
#define rall(v) (v).rbegin(),(v).rend()
#define SORT(v) sort((v).begin(),(v).end())
#define RSORT(v) sort((v).rbegin(),(v).rend())
#define EXIST(s,e) (find((s).begin(),(s).end(),(e))!=(s).end())
#define EXISTST(s,c) (((s).find(c))!=string::npos)
#define POSL(x,val) (lower_bound(x.begin(),x.end(),val)-x.begin())
#define POSU(x,val) (upper_bound(x.begin(),x.end(),val)-x.begin())
#define GEQ(x,val) (int)(x).size() - POSL((x),(val))
#define GREATER(x,val) (int)(x).size() - POSU((x),(val))
#define LEQ(x,val) POSU((x),(val))
#define LESS(x,val) POSL((x),(val))
#define SZV(a) int((a).size())
#define SZA(a) sizeof(a)/sizeof(a[0])
#define ZERO(a) memset(a,0,sizeof(a))
#define MINUS(a) memset(a,0xff,sizeof(a))
#define MEMINF(a) memset(a,0x3f,sizeof(a))
#define FILL(a,b) memset(a,b,sizeof(a))
#define UNIQUE(v) sort((v).begin(),(v).end());(v).erase(unique((v).begin(),(v).end()),(v).end())
struct abracadabra{
    abracadabra(){
        cin.tie(0); ios::sync_with_stdio(0);
        cout << fixed << setprecision(20);
        cerr << fixed << setprecision(5);
    };
} ABRACADABRA;

//TEMPLATE END---------------8<---------------8<---------------8<---------------8<---------------//

template<typename T> struct Edge {
    int from, to;
    T weight;
    Edge() : from(0), to(0), weight(0) {}
    Edge(int f, int t, T w) : from(f), to(t), weight(w) {}
};
template<typename T> using Edges = vector< Edge< T > >;
template<typename T> using Graph = vector< Edges< T > >;
template<typename T> void     add_edge(Graph< T > &g, int from, int to, T w = 1) { g[from].emplace_back(from, to, w); g[to].emplace_back(to, from, w); }
template<typename T> void      add_arc(Graph< T > &g, int from, int to, T w = 1) { g[from].emplace_back(from, to, w); }
template<typename T> void add_to_edges(Edges< T > &e, int from, int to, T w = 1) { e.emplace_back(from, to, w); }

/*
・グラフ用テンプレート
    > Dijkstra
    > BellmanFord
    > WarshallFloyd
    > Kruskal
[応用] 単一終点最短路問題は, すべての有向辺を逆向きに張り替えると, 単一始点最短路問題に帰着できる.
[使用例]
Graph<int> g(V);                // 頂点数V, 重さの型がintのグラフを宣言
add_edge(g, a, b, c);           // グラフgに, 始点a, 終点b, 重さcの無向辺を追加
add_arc(g, a, b, c);            // グラフgに, 始点a, 終点b, 重さcの有向辺を追加
add_to_edges(edges, a, b, c);   // 辺集合edgesに, 始点a, 終点b, 重さcの辺を追加
*/

template<typename T> using Matrix = vector< vector< T > >;
template<typename T> Matrix< T > WarshallFloyd(Graph< T > &g) {
    const auto INF = numeric_limits< T >::max() / 10;
    int vertex = g.size();
    Matrix< T > dist(vertex, vector< T >(vertex, INF));
    for (int i = 0; i < vertex; ++i) dist[i][i] = 0;
    for (int i = 0; i < vertex; ++i) {
        for (auto &e : g[i]) {
            dist[e.from][e.to] = min(dist[e.from][e.to], e.weight);
        }
    }
    for (int k = 0; k < vertex; ++k) {
        for (int i = 0; i < vertex; ++i) {
            for (int j = 0; j < vertex; ++j) {
                if (dist[i][k] != INF and dist[k][j] != INF) {
                    dist[i][j] = min(dist[i][j], dist[i][k] + dist[k][j]);
                }
            }
        }
    }
    return dist;
}
template<typename T> void add_edge_to_matrix(Matrix< T > &mat, int from, int to, T weight = 1) {
    const auto INF = numeric_limits< T >::max() / 10;
    mat[from][to] = mat[to][from] = min(mat[from][to], weight);
    int vertex = mat.size();
    for (int k : {from, to}) {
        for (int i = 0; i < vertex; ++i) {
            for (int j = 0; j < vertex; ++j) {
                if (mat[i][k] != INF and mat[k][j] != INF) {
                    mat[i][j] = min(mat[i][j], mat[i][k] + mat[k][j]);
                }
            }
        }
    }
}

/*
・ワーシャルフロイド法
    > O(V ^ 3) [V:頂点数] : 全点間最短路 (WarshallFloyd())
    > O(V ^ 2)           : 辺の追加 (add_edge_to_matrix())
[備考] 全点間最短路を求めるアルゴリズム
      負閉路が存在しているかの検出も可能
        -> ある頂点vからv自身への最短路が負なら負閉路が存在
[注意] 結果を足し合わせる際, INFの大きさに注意
[使用例]
auto wf = WarshallFloyd(g);         // グラフgにおける全点間最短路
add_edge_to_matrix(wf, a, b, c);    // 隣接行列wfに, 始点a, 終点b, 重さcの辺を追加して, wfを更新
*/

int H, W;
int f(int a, int b) {
    return a * W + b;
}

signed main() {

    cin >> H >> W;
    SV(string, board, H);

    Graph<int> g(H * W);

    REP(i, H) REP(j, W) {
        if (board[i][j] == '#') continue;
        REP(k, 4) {
            int ni = i + dx[k];
            int nj = j + dy[k];
            if (ni < 0 or ni >= H or nj < 0 or nj >= W) continue;
            if (board[ni][nj] == '#') continue;
            add_edge(g, f(i, j), f(ni, nj), 1);
        }
    }

    auto wf = WarshallFloyd(g);
    // debug(wf);

    int res = 0;
    REP(i, H) REP(j, W) {
        REP(a, H) REP(b, W) {
            if (wf[f(i, j)][f(a, b)] > INF / 100) continue;
            chmax(res, wf[f(i, j)][f(a, b)]);
        }
    }
    print(res);

}

Measured memory_bytes: 896