#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);
    }
}

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

    int H, W;
    scan(H, W);
    vector<vector<char>> maze(H, vector<char>(W));
    scan(maze);
    int ans        = 0;
    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;
    };

    rep(x1, W) {
        rep(y1, H) {
            rep(x2, W) {
                rep(y2, H) {
                    if (maze[y1][x1] == '#' || maze[y2][x2] == '#')
                        break;
                    vector<vector<int>> vis(H, vector<int>(W, false));
                    int mn   = INF32;
                    auto dfs = [&](auto f, int x, int y, int cnt,
                                   vector<vector<int>> vis) {
                        if (x == x2 && y == y2) {
                            chmin(mn, cnt);
                            return;
                        }
                        vis[y][x] = true;
                        rep(d, 4) {
                            int nx = x + dx[d];
                            int ny = y + dy[d];
                            if (exist(H, W, nx, ny) && !vis[ny][nx] &&
                                maze[ny][nx] == '.') {
                                f(f, nx, ny, cnt + 1, vis);
                            }
                        }
                        return;
                    };
                    dfs(dfs, x1, y1, 0, vis);
                    if (mn != INF32)
                        chmax(ans, mn);
                }
            }
        }
    }
    print(ans);

    return 0;
}