/* #region Head */

#include <bits/stdc++.h>
using namespace std;

using ll = long long;
using ull = unsigned long long;
using ld = long double;
using pll = pair<ll, ll>;
template <class T> using vc = vector<T>;
template <class T> using vvc = vc<vc<T>>;
using vll = vc<ll>;
using vvll = vvc<ll>;
using vld = vc<ld>;
using vvld = vvc<ld>;
using vs = vc<string>;
using vvs = vvc<string>;
template <class T, class U> using um = unordered_map<T, U>;
template <class T> using pq = priority_queue<T>;
template <class T> using pqa = priority_queue<T, vc<T>, greater<T>>;
template <class T> using us = unordered_set<T>;

#define REP(i, m, n) for (ll i = (m), i##_len = (ll)(n); i < i##_len; ++(i))
#define REPM(i, m, n) for (ll i = (m), i##_max = (ll)(n); i <= i##_max; ++(i))
#define REPR(i, m, n) for (ll i = (m), i##_min = (ll)(n); i >= i##_min; --(i))
#define REPD(i, m, n, d) for (ll i = (m), i##_len = (ll)(n); i < i##_len; i += (d))
#define REPMD(i, m, n, d) for (ll i = (m), i##_max = (ll)(n); i <= i##_max; i += (d))
#define REPI(itr, ds) for (auto itr = ds.begin(); itr != ds.end(); itr++)
#define ALL(x) begin(x), end(x)
#define SIZE(x) ((ll)(x).size())
#define PERM(c)                                                                                                        \
    sort(ALL(c));                                                                                                      \
    for (bool c##p = 1; c##p; c##p = next_permutation(ALL(c)))
#define UNIQ(v) v.erase(unique(ALL(v)), v.end());

#define endl '\n'
#define sqrt sqrtl
#define floor floorl
#define log2 log2l

constexpr ll INF = 1'010'000'000'000'000'017LL;
constexpr ll MOD = 1'000'000'007LL; // 1e9 + 7
constexpr ld EPS = 1e-12;
constexpr ld PI = 3.14159265358979323846;

template <typename T> istream &operator>>(istream &is, vc<T> &vec) { // vector 入力
    for (T &x : vec) is >> x;
    return is;
}
template <typename T> ostream &operator<<(ostream &os, vc<T> &vec) { // vector 出力 (for dump)
    os << "{";
    REP(i, 0, SIZE(vec)) os << vec[i] << (i == i_len - 1 ? "" : ", ");
    os << "}";
    return os;
}
template <typename T> ostream &operator>>(ostream &os, vc<T> &vec) { // vector 出力 (inline)
    REP(i, 0, SIZE(vec)) os << vec[i] << (i == i_len - 1 ? "\n" : " ");
    return os;
}

template <typename T, typename U> istream &operator>>(istream &is, pair<T, U> &pair_var) { // pair 入力
    is >> pair_var.first >> pair_var.second;
    return is;
}
template <typename T, typename U> ostream &operator<<(ostream &os, pair<T, U> &pair_var) { // pair 出力
    os << "(" << pair_var.first << ", " << pair_var.second << ")";
    return os;
}

// map, um, set, us 出力
template <class T> ostream &out_iter(ostream &os, T &map_var) {
    os << "{";
    REPI(itr, map_var) {
        os << *itr;
        auto itrcp = itr;
        if (++itrcp != map_var.end()) os << ", ";
    }
    return os << "}";
}
template <typename T, typename U> ostream &operator<<(ostream &os, map<T, U> &map_var) { return out_iter(os, map_var); }
template <typename T, typename U> ostream &operator<<(ostream &os, um<T, U> &map_var) {
    os << "{";
    REPI(itr, map_var) {
        auto [key, value] = *itr;
        os << "(" << key << ", " << value << ")";
        auto itrcp = itr;
        if (++itrcp != map_var.end()) os << ", ";
    }
    os << "}";
    return os;
}
template <typename T> ostream &operator<<(ostream &os, set<T> &set_var) { return out_iter(os, set_var); }
template <typename T> ostream &operator<<(ostream &os, us<T> &set_var) { return out_iter(os, set_var); }
template <typename T> ostream &operator<<(ostream &os, pq<T> &pq_var) {
    pq<T> pq_cp(pq_var);
    os << "{";
    if (!pq_cp.empty()) {
        os << pq_cp.top(), pq_cp.pop();
        while (!pq_cp.empty()) os << ", " << pq_cp.top(), pq_cp.pop();
    }
    return os << "}";
}

// dump
#define DUMPOUT cerr
void dump_func() { DUMPOUT << endl; }
template <class Head, class... Tail> void dump_func(Head &&head, Tail &&... tail) {
    DUMPOUT << head;
    if (sizeof...(Tail) > 0) DUMPOUT << ", ";
    dump_func(move(tail)...);
}

// chmax (更新「される」かもしれない値が前)
template <typename T, typename U, typename Comp = less<>> bool chmax(T &xmax, const U &x, Comp comp = {}) {
    if (comp(xmax, x)) {
        xmax = x;
        return true;
    }
    return false;
}

// chmin (更新「される」かもしれない値が前)
template <typename T, typename U, typename Comp = less<>> bool chmin(T &xmin, const U &x, Comp comp = {}) {
    if (comp(x, xmin)) {
        xmin = x;
        return true;
    }
    return false;
}

// ローカル用
#define DEBUG_

#ifdef DEBUG_
#define DEB
#define dump(...)                                                                                                      \
    DUMPOUT << "  " << string(#__VA_ARGS__) << ": "                                                                    \
            << "[" << to_string(__LINE__) << ":" << __FUNCTION__ << "]" << endl                                        \
            << "    ",                                                                                                 \
        dump_func(__VA_ARGS__)
#else
#define DEB if (false)
#define dump(...)
#endif

struct AtCoderInitialize {
    static constexpr int IOS_PREC = 15;
    static constexpr bool AUTOFLUSH = false;
    AtCoderInitialize() {
        ios_base::sync_with_stdio(false), cin.tie(nullptr), cout.tie(nullptr);
        cout << fixed << setprecision(IOS_PREC);
        if (AUTOFLUSH) cout << unitbuf;
    }
} ATCODER_INITIALIZE;

void Yn(bool p) { cout << (p ? "Yes" : "No") << endl; }
void YN(bool p) { cout << (p ? "YES" : "NO") << endl; }

/* #endregion */

/* #region Dist */

// 距離用整数
// INF を -1 と出力する整数
// 不定形にならないと仮定して演算する
struct Dist {
    ll x;
    Dist(ll x = 0) : x(x) {}
    Dist &operator+=(const Dist a) {
        if (x == INF || a.x == INF) return *this;
        if (x == -INF || a.x == -INF) return *this;
        x += a.x;
        return *this;
    }
    Dist &operator-=(const Dist a) {
        if (x == INF || -a.x == INF) return *this;
        if (x == -INF || -a.x == -INF) return *this;
        x -= a.x;
        return *this;
    }
    Dist &operator*=(const Dist a) {
        if (x == INF || x == -INF) {
            if (a.x < 0) x *= -1;
            return *this;
        } else if (a.x == INF || a.x == -INF) {
            x = (x > 0) ? a.x : -a.x;
            return *this;
        }
        x *= a.x;
        return *this;
    }
    Dist operator+(const Dist a) const {
        Dist res(*this);
        return res += a;
    }
    Dist operator-(const Dist a) const {
        Dist res(*this);
        return res -= a;
    }
    Dist operator*(const Dist a) const {
        Dist res(*this);
        return res *= a;
    }
    // O(log(t))
    Dist pow(ll t) const {
        if (!t) return 1;
        Dist a = pow(t >> 1); // ⌊t/2⌋ 乗
        a *= a;               // ⌊t/2⌋*2 乗
        if (t & 1)            // ⌊t/2⌋*2 == t-1 のとき
            a *= *this;       // ⌊t/2⌋*2+1 乗 => t 乗
        return a;
    }

    Dist &operator/=(const Dist a) {
        if (x == INF || x == -INF) {
            if (a.x < 0) x *= -1;
            return *this;
        } else if (a.x == INF || a.x == -INF) {
            x = 0;
            return *this;
        }
        x /= a.x;
        return *this;
    }
    Dist operator/(const Dist a) const {
        Dist res(*this);
        return res /= a;
    }
    bool operator==(const Dist a) const { return x == a.x; }
    bool operator!=(const Dist a) const { return x != a.x; }
    bool operator==(const ll a) const { return x == a; }
    bool operator!=(const ll a) const { return x != a; }

    bool operator<(const Dist a) const { return x < a.x; }
    bool operator>(const Dist a) const { return x > a.x; }
    bool operator<=(const Dist a) const { return x <= a.x; }
    bool operator>=(const Dist a) const { return x >= a.x; }

    Dist operator+() const { return *this; }
    Dist operator-() const { return Dist(-x); }

    // Dist 入力
    friend istream &operator>>(istream &is, Dist &x) {
        is >> x.x;
        return is;
    }

    // Dist 出力
    friend ostream &operator<<(ostream &os, Dist x) {
        if (x.x == INF || x.x == -INF)
            os << -1;
        else
            os << x.x;
        return os;
    }
};

/* #endregion */

// Problem
void solve() {
    using CoordRow = int;
    using CoordCol = int;
    char wall = '#';

    int h, w;
    cin >> h >> w;
    vc<vc<char>> s(h, vc<char>(w));
    cin >> s;

    Dist ans = 0;

    REP(sRow, 0, h) REP(sCol, 0, w) {
        if (s[sRow][sCol] == wall) continue;

        // ここからBFS本体
        const int dCol[4] = {1, 0, -1, 0};
        const int dRow[4] = {0, 1, 0, -1};
        using node = pair<CoordRow, CoordCol>;

        vc<vc<Dist>> dist(h, vc<Dist>(w, INF)); // 最小回数
        dist[sRow][sCol] = 0;
        queue<node> que;
        que.emplace(sRow, sCol);

        while (!que.empty()) {
            auto [curRow, curCol] = que.front();
            que.pop();
            Dist curDist = dist[curRow][curCol];

            // cuerrent から辿れる頂点をすべて調べる
            REP(dir, 0, 4) {
                ll nextRow = curRow + dRow[dir];
                ll nextCol = curCol + dCol[dir];
                if (nextRow < 0 || nextRow >= h || nextCol < 0 || nextCol >= w) continue;
                if (s[nextRow][nextCol] == wall) continue;
                if (dist[nextRow][nextCol] != INF) continue; // 訪問済み（BFS）

                Dist nextDist = curDist + 1;
                // if (chmin(dist[nextRow][nextCol], nextDist)) que.emplace(nextRow, nextCol); //
                // ↑この書き方はダイクストラっぽい
                dist[nextRow][nextCol] = nextDist;
                que.emplace(nextRow, nextCol);
            }
        }
        // BFS 終了，dist が埋まった
        Dist ma = 0;
        REP(row, 0, h) REP(col, 0, w) if (dist[row][col] != INF) chmax(ma, dist[row][col]);
        chmax(ans, ma);
    }
    cout << ans << endl;
}

// entry point
int main() {
    solve();
    return 0;
}


Measured memory_bytes: 3588