#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 r, c;
    cin>>r>>c;
    vector<Vs> map(r+2, Vs(c+2));

    rep(i, r) {
        string tmp;
        cin>>tmp;
        rep(j, tmp.length()) {
            map[i+1][j+1] = tmp[j];
        }
    }

    ll res = 0;
    REP(sh,1,r+1){
        REP(sw,1,c+1){
            VV score(r+2, Vec(c+2, INF));
            ll ans = 0;
            if(map[sh][sw]=="."){
                queue<tup> q;
                q.push(tup(0, sh, sw));
                while (!q.empty()) {
                    tup &now = q.front();
                    if(now.fi < score[now.se][now.th]) {
                        score[now.se][now.th] = now.fi;
                        if(map[now.se+1][now.th] == ".") {
                            q.push(tup(now.fi+1, now.se+1, now.th));
                        }
                        if(map[now.se-1][now.th] == ".") {
                            q.push(tup(now.fi+1, now.se-1, now.th));
                        }
                        if(map[now.se][now.th+1] == ".") {
                            q.push(tup(now.fi+1, now.se, now.th+1));
                        }
                        if(map[now.se][now.th-1] == ".") {
                            q.push(tup(now.fi+1, now.se, now.th-1));
                        }
                    }
                    q.pop();
                }
            }
            REP(th,1,r+1){
                REP(tw,1,c+1) {
                    if(score[th][tw]!=INF){
                        chmax(ans,score[th][tw]);
                    }
                }
            }
            chmax(res, ans);
        }
    }

    cout<<res<<endl;
    return 0;
}


Measured memory_bytes: 256