#include <bits/stdc++.h>
#define sort_(a) sort(a.begin(), a.end())
#define rsort(a) sort(a.rbegin(), a.rend())
#define rep(i, n) for (int i = 0; i < n; i++)
#define rrep(i, n) for (int i = n - 1; 0 <= i; i--)
#define for_(i, start, end) for (int i = start; i < end; i++)
#define rfor(i, start, end) for (int i = start - 1; 0 <= i; i--)
#define all(a) a.begin(), a.end()
using ll = long long;
using ld = long double;
using namespace std;
constexpr int INF32       = 1'050'000'000;
constexpr long long INF64 = 4'000'000'000'000'000'000;
constexpr int MOD7        = 1'000'000'007;
constexpr int MOD53       = 998'244'353;
constexpr double PI       = 3.14159265358979323846;
template <class T> inline bool chmin(T &a, T b) {
    if (a > b) {
        a = b;
        return true;
    }
    return false;
}
template <class T> inline bool chmax(T &a, T b) {
    if (a < b) {
        a = b;
        return true;
    }
    return false;
}
void print() { std::cout << '\n'; }
template <class H, class... T> void print(H &&head, T &&... args) {
    std::cout << head;
    sizeof...(args) == 0 ? std::cout << "" : std::cout << ' ';
    print(std::forward<T>(args)...);
}
template <class T> void print(std::vector<T> &v) {
    for (int i = 0; i < v.size(); i++) {
        std::cout << v[i];
        i == v.size() - 1 ? std::cout << '\n' : std::cout << ' ';
    }
}
template <class T> void print(std::vector<std::vector<T>> &v) {
    for (int i = 0; i < v.size(); i++) {
        for (int j = 0; j < v[i].size(); j++) {
            std::cout << v[i][j];
            j == v[i].size() - 1 ? std::cout << '\n' : std::cout << ' ';
        }
    }
}
void scan() {}
template <class H, class... T> void scan(H &&head, T &&... args) {
    std::cin >> head;
    scan(std::forward<T>(args)...);
}
template <class T> void scan(std::vector<T> &v) {
    for (auto &&i : v) {
        scan(i);
    }
}

class Warshall_floyd {
  private:
    int vertex;
    const long long INF = 4'000'000'000'000'000'000;
    vector<vector<long long>> cost;

  public:
    Warshall_floyd(int N) {
        vertex = N;
        cost.resize(N, vector<long long>(N, INF));
        for (int i = 0; i < vertex; i++) {
            cost[i][i] = 0;
        }
    }
    void directed_update(int x, int y, long long weight) {
        cost[x][y] = weight;
    }
    void undirected_update(int x, int y, long long weight) {
        cost[x][y] = weight;
        cost[y][x] = weight;
    }
    void calc() {
        for (int k = 0; k < vertex; k++) {
            for (int i = 0; i < vertex; i++) {
                for (int j = 0; j < vertex; j++) {
                    if (cost[i][k] != INF && cost[k][j] != INF)
                        cost[i][j] = min(cost[i][j], cost[i][k] + cost[k][j]);
                }
            }
        }
    }
    // pair 型で返す。first が距離で second が実現可能か
    pair<long long, bool> dist(int x, int y) {
        pair<long long, bool> ans;
        if (cost[x][y] == INF) {
            ans.first  = 0;
            ans.second = false;
        } else {
            ans.first  = cost[x][y];
            ans.second = true;
        }
        return ans;
    }
    bool is_negative() {
        for (int i = 0; i < vertex; i++) {
            if (cost[i][i] < 0) {
                return true;
            }
        }
        return false;
    }
};

void debug() { std::cerr << '\n'; }
template <class H, class... T> void debug(H &&head, T &&... args) {
    std::cerr << head;
    sizeof...(args) == 0 ? std::cerr << "" : std::cerr << ' ';
    debug(std::forward<T>(args)...);
}
template <class T> void debug(std::vector<T> &v) {
    for (int i = 0; i < v.size(); i++) {
        std::cerr << v[i];
        i == v.size() - 1 ? std::cerr << '\n' : std::cerr << ' ';
    }
}
template <class T> void debug(std::vector<std::vector<T>> &v) {
    for (int i = 0; i < v.size(); i++) {
        for (int j = 0; j < v[i].size(); j++) {
            std::cerr << v[i][j];
            j == v[i].size() - 1 ? std::cerr << '\n' : std::cerr << ' ';
        }
    }
}

int main() {
    cin.tie(0);
    ios::sync_with_stdio(false);

    int H, W;
    scan(H, W);
    vector<string> maze(H);
    scan(maze);
    vector<int> dx = {-1, 1, 0, 0};
    vector<int> dy = {0, 0, -1, 1};
    auto exist     = [](int H, int W, int nx, int ny) -> bool {
        if (nx < 0 || W <= nx || ny < 0 || H <= ny)
            return false;
        else
            return true;
    };
    auto wf = Warshall_floyd(H * W + 1);
    map<pair<int, int>, int> id;
    int cnt = 0;
    rep(y, H) {
        rep(x, W) { id[{x, y}] = cnt++; }
    }

    rep(y, H) {
        rep(x, W) {
            if (maze[y][x] == '#')
                continue;
            rep(i, 4) {
                int nx = x + dx[i];
                int ny = y + dy[i];
                if (exist(H, W, nx, ny) && maze[ny][nx] == '.') {
                    wf.undirected_update(id[{x, y}], id[{nx, ny}], 1);
                }
            }
        }
    }
    wf.calc();
    int ans = 0;
    rep(sy, H) {
        rep(sx, W) {
            rep(gy, H) {
                rep(gx, W) {
                    // if (maze[sy][sx] == '#' || maze[gy][gx] == '#')
                    //     break;
                    int start = id[{sx, sy}];
                    int goal  = id[{gx, gy}];
                    auto go   = wf.dist(start, goal);
                    if (go.second) {
                        int dist = go.first;
                        chmax(ans, dist);
                    }
                }
            }
        }
    }
    print(ans);

    return 0;
}

Measured memory_bytes: 1536