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

#define int long long
#define double long double
#define rep(i, n) for (int i = 0; i < (n); i++)
#define reps(i, n) for (int i = 1; i <= (n); i++)
#define all(x) begin(x), end(x)
#define uniq(x) (x).erase(unique(all(x)), end(x))
#define bit(n) (1LL << (n))
#define dump(x) cerr << #x " = " << (x) << endl
using vint = vector<int>;
using vvint = vector<vint>;
using pint = pair<int, int>;
using vpint = vector<pint>;
template<typename T> using priority_queue_rev = priority_queue<T, vector<T>, greater<T>>;
constexpr double pi = 3.1415926535897932384626433832795028L;
constexpr int dy[8] = {0, 1, 0, -1, 1, 1, -1, -1};
constexpr int dx[8] = {1, 0, -1, 0, 1, -1, -1, 1};
int gcd(int a, int b) {
    while (b) { swap(a %= b, b); }
    return a;
}
int lcm(int a, int b) { return a / gcd(a, b) * b; }
template<typename T> void fin(T mes) {
    cout << mes << endl;
    exit(0);
}
template<typename T, typename U> bool chmax(T &a, const U &b) {
    if (a < b) {
        a = b;
        return true;
    }
    return false;
}
template<typename T, typename U> bool chmin(T &a, const U &b) {
    if (b < a) {
        a = b;
        return true;
    }
    return false;
}
template<typename T, typename U> ostream &operator<<(ostream &os, const pair<T, U> &rhs) {
    os << "(" << rhs.first << ", " << rhs.second << ")";
    return os;
}
template<typename T> ostream &operator<<(ostream &os, const vector<T> &rhs) {
    os << "{";
    for (auto itr = rhs.begin(); itr != rhs.end(); itr++) {
        os << *itr << (next(itr) != rhs.end() ? ", " : "");
    }
    os << "}";
    return os;
}
struct setup {
    static constexpr int PREC = 20;
    setup() {
        cout << fixed << setprecision(PREC);
        cerr << fixed << setprecision(PREC);
    };
} setup;

template<typename T>
struct warshall_floyd {
    int n;
    T zero, inf;
    std::vector<std::vector<int>> graph;
    warshall_floyd(int n, T zero = T(), T inf = std::numeric_limits<T>::max())
        : n(n), zero(zero), inf(inf), graph(n, std::vector<int>(n, inf)) {
        for (int i = 0; i < n; i++) { graph[i][i] = zero; }
    }
    void add_edge(int from, int to, T cost) { graph[from][to] = cost; }
    std::vector<std::vector<T>> get_dist() const {
        std::vector<std::vector<T>> dist(graph);
        for (int k = 0; k < n; k++) {
            for (int i = 0; i < n; i++) {
                for (int j = 0; j < n; j++) {
                    if (dist[i][k] == inf || dist[k][j] == inf) { continue; }
                    T next_dist = dist[i][k] + dist[k][j];
                    if (next_dist < dist[i][j]) { dist[i][j] = next_dist; }
                }
            }
        }
        return dist;
    }
};

int H, W;
string S[22];
signed main() {
    cin >> H >> W;
    rep(i, H) { cin >> S[i]; }
    warshall_floyd<int> wf(H * W);
    rep(i, H) {
        for (int j = 0; j < W - 1; j++) {
            if (S[i][j] == '.' && S[i][j + 1] == '.') {
                wf.add_edge(i * W + j, i * W + (j + 1), 1);
                wf.add_edge(i * W + (j + 1), i * W + j, 1);
            }
        }
    }
    rep(j, W) {
        for (int i = 0; i < H - 1; i++) {
            if (S[i][j] == '.' && S[i + 1][j] == '.') {
                wf.add_edge(i * W + j, (i + 1) * W + j, 1);
                wf.add_edge((i + 1) * W + j, i * W + j, 1);
            }
        }
    }
    vvint dist = wf.get_dist();
    int ans = 0;
    rep(i, H * W) {
        rep(j, H * W) {
            if (dist[i][j] != wf.inf) { chmax(ans, dist[i][j]); }
        }
    }
    cout << ans << endl;
}

Measured memory_bytes: 2816