#include <bits/stdc++.h>

using namespace std;

typedef long long ll;
typedef pair<ll, ll> P;
typedef pair<int, int> Pi;
typedef vector<ll> Vec;
typedef vector<int> Vi;
typedef vector<string> Vs;
typedef vector<P> VP;
typedef vector<vector<ll>> VV;
typedef vector<vector<int>> VVi;

#define REP(i, a, b) for(ll i=(a); i<(b); i++)
#define PER(i, a, b) for(ll i=(a); i>=(b); i--)
#define rep(i, n) REP(i, 0, n)
#define per(i, n) PER(i, n, 0)
const ll INF = 1e18+18;
const ll MAX = 100005;
const ll MOD = 1000000007;
#define Yes(n) cout << ((n) ? "Yes" : "No") << endl;
#define YES(n) cout << ((n) ? "YES" : "NO") << endl;
#define ALL(v) v.begin(), v.end()
#define rALL(v) v.rbegin(), v.rend()
#define pb(x) push_back(x)
#define mp(a, b) make_pair(a,b)
#define Each(a, b) for(auto &a :b)
#define REPM(i, mp) for (auto i = mp.begin(); i != mp.end(); ++i)
#define dbg(x_) cerr << #x_ << ":" << x_ << endl;
#define dbgmap(mp) cerr << #mp << ":"<<endl; for (auto i = mp.begin(); i != mp.end(); ++i) { cerr << i->first <<":"<<i->second << endl;}
#define dbgarr(n, m, arr) rep(i,n){rep(j,m){cerr<<arr[i][j]<<" ";}cerr<<endl;}
#define dbgdp(n, arr) rep(i,n){cerr<<arr[i]<<" ";}cerr<<endl;
#define Uniq(v) v.erase(unique(v.begin(), v.end()), v.end())
#define fi first
#define se second

template<class T>
bool chmax(T &a, const T &b) {
    if(a < b) {
        a = b;
        return true;
    }
    return false;
}

template<class T>
bool chmin(T &a, const T &b) {
    if(b < a) {
        a = b;
        return true;
    }
    return false;
}

template<typename T1, typename T2>
ostream &operator<<(ostream &s, const pair<T1, T2> &p) { return s<<"("<<p.first<<", "<<p.second<<")"; }

// vector
template<typename T>
ostream &operator<<(ostream &s, const vector<T> &v) {
    int len = v.size();
    for(int i = 0; i < len; ++i) {
        s<<v[i];
        if(i < len-1) s<<"	";
    }
    return s;
}

// 2 dimentional vector
template<typename T>
ostream &operator<<(ostream &s, const vector<vector<T> > &vv) {
    int len = vv.size();
    for(int i = 0; i < len; ++i) {
        s<<vv[i]<<endl;
    }
    return s;
}


struct tup {
    ll fi, se, th;

    tup() {};

    tup(ll a, ll b, ll c) : fi(a), se(b), th(c) {};
};

int main() {
    cin.tie(0);
    ios::sync_with_stdio(false);
    cout<<std::setprecision(10);

    ll h,w;
    cin>>h>>w;

    Vs grid(h);
    rep(i,h) cin>>grid[i];

    ll ans = 0;
    Vec di(4);
    di = {1,0,-1,0};
    Vec dj(4);
    dj = {0,1,0,-1};
    rep(si,h){
        rep(sj,w){
            if(grid[si][sj]!='.')continue;
            VV dist(h,Vec(w));
            rep(i,h)rep(j,w){
                dist[i][j] = INF;
            }
            queue<P> q;
            q.push(mp(si,sj));
            dist[si][sj] = 0;
            while (!q.empty()){
                P p = q.front();
                q.pop();
                rep(k,4){
                    ll ni = p.fi+di[k];
                    ll nj = p.se+dj[k];
                    if(ni==-1||ni==h||nj==-1||nj==w)continue;
                    if(grid[ni][nj]!='.')continue;
                    if(dist[ni][nj]>dist[p.fi][p.se]+1){
                        dist[ni][nj] = dist[p.fi][p.se]+1;
                        q.push(mp(ni,nj));
                    }
                }
            }
            ll tmp = 0;
            rep(i,h){
                rep(j,w){
                    if(dist[i][j]==INF)continue;
                    chmax(tmp,dist[i][j]);
                }
            }
            chmax(ans,tmp);
        }
    }

    cout<<ans<<endl;
    return 0;
}


Measured memory_bytes: 256