// >>> TEMPLATES
#include <bits/stdc++.h>
using namespace std;
using ll = long long;
using ld = long double;
#define int ll
#define double ld
#define rep(i,n) for (int i = 0; i < (int)(n); i++)
#define rep1(i,n) for (int i = 1; i <= (int)(n); i++)
#define repR(i,n) for (int i = (int)(n)-1; i >= 0; i--)
#define rep1R(i,n) for (int i = (int)(n); i >= 1; i--)
#define loop(i,a,B) for (int i = a; i B; i++)
#define loopR(i,a,B) for (int i = a; i B; i--)
#define all(x) (x).begin(), (x).end()
#define allR(x) (x).rbegin(), (x).rend()
#define pb push_back
#define eb emplace_back
#define mp make_pair
#define fst first
#define snd second
#define INF (numeric_limits<int>::max()/2-1)
#ifdef LOCAL
#include "debug.hpp"
#define dump(...) cerr << "[" << __LINE__ << ":" << __FUNCTION__ << "] ", dump_impl(#__VA_ARGS__, __VA_ARGS__)
#define say(x) cerr << "[" << __LINE__ << ":" << __FUNCTION__ << "] " << x << endl
#define debug if (1)
#else
#define dump(...) (void)(0)
#define say(x) (void)(0)
#define debug if (0)
#endif
template <class T> using pque_max = priority_queue<T>;
template <class T> using pque_min = priority_queue<T, vector<T>, greater<T> >;
template <class T, class = typename T::iterator, class = typename enable_if<!is_same<T, string>::value>::type>
ostream& operator<<(ostream& os, T const& v) { bool f = true; for (auto const& x : v) os << (f ? "" : " ") << x, f = false; return os; }
template <class T, class = typename T::iterator, class = typename enable_if<!is_same<T, string>::value>::type>
istream& operator>>(istream& is, T &v) { for (auto& x : v) is >> x; return is; }
template <class T, class S> istream& operator>>(istream& is, pair<T,S>& p) { return is >> p.first >> p.second; }
struct IOSetup { IOSetup() { cin.tie(nullptr); ios::sync_with_stdio(false); cout << fixed << setprecision(15); } } iosetup;
template <class F> struct FixPoint : private F {
    constexpr FixPoint(F&& f) : F(forward<F>(f)) {}
    template <class... T> constexpr auto operator()(T&&... x) const { return F::operator()(*this, forward<T>(x)...); }
};
struct MakeFixPoint {
    template <class F> constexpr auto operator|(F&& f) const { return FixPoint<F>(forward<F>(f)); }
};
#define MFP MakeFixPoint()|
#define def(name, ...) auto name = MFP [&](auto &&name, __VA_ARGS__)
template <class T, size_t d> struct vec_impl {
    using type = vector<typename vec_impl<T,d-1>::type>;
    template <class... U> static type make_v(size_t n, U&&... x) { return type(n, vec_impl<T,d-1>::make_v(forward<U>(x)...)); }
};
template <class T> struct vec_impl<T,0> { using type = T; static type make_v(T const& x = {}) { return x; } };
template <class T, size_t d = 1> using vec = typename vec_impl<T,d>::type;
template <class T, size_t d = 1, class... Args> auto make_v(Args&&... args) { return vec_impl<T,d>::make_v(forward<Args>(args)...); }
template <class T> void quit(T const& x) { cout << x << endl; exit(0); }
template <class T> constexpr bool chmin(T& x, T const& y) { if (x > y) { x = y; return true; } return false; }
template <class T> constexpr bool chmax(T& x, T const& y) { if (x < y) { x = y; return true; } return false; }
template <class It> constexpr auto sumof(It b, It e) { return accumulate(b,e,typename iterator_traits<It>::value_type{}); }
template <class T> int sz(T const& x) { return x.size(); }
// <<<

int32_t main() {
    int n,m; cin >> n >> m;
    vector<string> s(n); cin >> s;

    int cnt = 0;
    map<pair<int,int>,int > id;
    rep (i,n) rep (j,m) if (s[i][j] == '.') id[mp(i,j)] = cnt++;
    dump(cnt);

    vector<vector<int> > g(cnt);
    const int dx[] = { 1, 0, -1, 0 };
    const int dy[] = { 0, 1, 0, -1 };
    rep (i,n) rep (j,m) if (s[i][j] == '.') {
        rep (k,4) {
            int ni = i + dx[k];
            int nj = j + dy[k];
            if (0 <= ni && ni < n && 0 <= nj && nj < m
                && s[ni][nj] == '.') {
                int x = id[mp(i,j)];
                int y = id[mp(ni,nj)];
                g[x].eb(y);
            }
        }
    }

    int ma = 0;
    rep (t,cnt) {
        vector<int> d(cnt,INF);
        queue<int> q;
        q.push(t);
        d[t] = 0;
        while (!q.empty()) {
            int x = q.front(); q.pop();
            for (int y : g[x]) {
                if (chmin(d[y], d[x]+1)) {
                    q.push(y);
                }
            }
        }
        chmax(ma, *max_element(all(d)));
        debug {
            rep (i,n) {
                rep (j,m) {
                    if (s[i][j] == '.') {
                        cerr << d[id[mp(i,j)]] << " ";
                    } else {
                        cerr << "# ";
                    }
                }
                cerr << endl;
            }
            cerr << endl;
        }

    }

    cout << ma << endl;
}


Measured memory_bytes: 384