#include "bits/stdc++.h"
#pragma GCC optimize("Ofast")

// Begin Header {{{
using namespace std;

#ifndef DEBUG
#define dump(...)
#endif

#define all(x) x.begin(), x.end()
#define rall(x) x.rbegin(), x.rend()
#define rep(i, n) for (intmax_t i = 0, i##_limit = (n); i < i##_limit; ++i)
#define reps(i, b, e) for (intmax_t i = (b), i##_limit = (e); i <= i##_limit; ++i)
#define repr(i, b, e) for (intmax_t i = (b), i##_limit = (e); i >= i##_limit; --i)
#define var(Type, ...) Type __VA_ARGS__; input(__VA_ARGS__)

constexpr size_t    operator""_zu(unsigned long long value) { return value; };
constexpr intmax_t  operator""_jd(unsigned long long value) { return value; };
constexpr uintmax_t operator""_ju(unsigned long long value) { return value; };

constexpr int INF = 0x3f3f3f3f;
constexpr intmax_t LINF = 0x3f3f3f3f3f3f3f3f_jd;

namespace moke {

template <class T> using MaxHeap = priority_queue<T, vector<T>, less<T>>;
template <class T> using MinHeap = priority_queue<T, vector<T>, greater<T>>;

inline void input() {}
template <class Head, class... Tail> inline void input(Head&& head, Tail&&... tail) {
    cin >> head;
    input(forward<Tail>(tail)...);
}

inline void outs() { cout << "\n"; }
template <class Head, class... Tail> inline void outs(Head&& head, Tail&&... tail) {
    cout << head << (sizeof...(tail) ? " " : "");
    outs(forward<Tail>(tail)...);
}

template <class T> inline void outs(vector<T> &vec) {
    for (auto &e : vec) cout << e << " \n"[&e == &vec.back()];
}

template <class T> inline void outs(vector<vector<T>> &df) {
    for (auto &vec : df) outs(vec);
}

inline void outl() { cout << "\n"; }
template <class Head, class... Tail> inline void outl(Head&& head, Tail&&... tail) {
    cout << head << (sizeof...(tail) ? "\n" : "");
    outl(forward<Tail>(tail)...);
}

template <class T> inline void outl(vector<T> &vec) {
    for (auto &e : vec) cout << e << "\n";
}

inline void outn() {}
template <class Head, class... Tail> inline void outn(Head&& head, Tail&&... tail) {
    cout << head;
    outn(forward<Tail>(tail)...);
}

template <class T> inline void outn(vector<T> &vec) {
    for (auto &e : vec) cout << e;
}

template <class T> inline vector<T> make_v(const T &initValue, size_t sz) {
    return vector<T>(sz, initValue);
}

template <class T, class... Args> inline auto make_v(const T &initValue, size_t sz, Args... args) {
    return vector<decltype(make_v<T>(initValue, args...))>(sz, make_v<T>(initValue, args...));
}

template <class A, class B> inline bool chmax(A &a, const B &b) noexcept {
    return b > a && (a = b, true);
}

template <class A, class B> inline bool chmin(A &a, const B &b) noexcept {
    return b < a && (a = b, true);
}

template <class A, class B> inline common_type_t<A, B> min(const A &a, const B &b) noexcept {
    return a < b ? a : b;
}

template <class A, class B, class... Args>
inline common_type_t<A, B, Args...> min(const A &a, const B &b, const Args&... args) noexcept {
    return moke::min(a < b ? a : b, args...);
}

template <class A, class B> inline common_type_t<A, B> max(const A &a, const B &b) noexcept {
    return a > b ? a : b;
}

template <class A, class B, class... Args>
inline common_type_t<A, B, Args...> max(const A &a, const B &b, const Args&... args) noexcept {
    return moke::max(a > b ? a : b, args...);
}

template <class A, class B> inline common_type_t<A, B> diff(const A &a, const B &b) noexcept {
    return a < b ? b - a : a - b;
}

} // namespace moke

// }}} End Header

namespace moke {

constexpr int dy[] = {0, 1, 0, -1, -1, 1, 1, -1};
constexpr int dx[] = {1, 0, -1, 0, 1, 1, -1, -1};

inline bool inside(int y, int x, int H, int W) {
    return (y >= 0 && x >= 0 && y < H && x < W);
}

int main_() {
    ios_base::sync_with_stdio(false);
    cin.tie(nullptr);

    var(int, H, W);

    vector<string> mat(H);
    for (auto &s: mat) input(s);

    intmax_t maxv = 0;
    rep(sy, H) rep(sx, W) {
        if (mat[sy][sx] == '.') {
            queue<pair<int, int>> que;
            que.emplace(sy, sx);

            auto dist = make_v<int>(INF, H, W);
            dist[sy][sx] = 0;

            while (!que.empty()) {
                int y, x;
                tie(y, x) = que.front();
                que.pop();
                rep(i, 4) {
                    int ny = y + dy[i];
                    int nx = x + dx[i];
                    if (inside(ny, nx, H, W) && mat[ny][nx] != '#' && chmin(dist[ny][nx], dist[y][x] + 1)) {
                        que.emplace(ny, nx);
                    }
                }
            }

            rep(y, H) rep(x, W) {
                if (mat[y][x] != '#' && dist[y][x] != INF) {
                    chmax(maxv, dist[y][x]);
                }
            }

        }
    }

    outl(maxv);

    return 0;
}

} // namespace moke

signed main() { // {{{
    moke::main_();
    return 0;
} // }}}


Measured memory_bytes: 256