#include "bits/stdc++.h"
// Begin Header {{{
#define all(x) (x).begin(), (x).end()
#define rep(i, s, n) for (i64 i = (s), i##_limit = (n); i < i##_limit; ++i)
#define repr(i, s, t) for (i64 i = (s), i##_limit = (t); i >= i##_limit; --i)
#define var(type, ...) \
    type __VA_ARGS__;  \
    read(__VA_ARGS__);
#ifndef DBG
#define dump(...)
#endif
using namespace std;
using i64 = int_fast64_t;
using pii = pair<i64, i64>;
template <class T, class U>
inline bool chmax(T &a, const U &b) {
    return b > a && (a = b, true);
}
template <class T, class U>
inline bool chmin(T &a, const U &b) {
    return b < a && (a = b, true);
}
constexpr int INF = 0x3f3f3f3f;
constexpr i64 LINF = 0x3f3f3f3f3f3f3f3fLL;

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

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

template <class T>
inline istream &operator>>(istream &is, vector<T> &vec) {
    for (auto &e : vec) is >> e;
    return is;
}

inline void read() {}

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

inline void print() { cout << "\n"; }

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

template <class T>
inline ostream &operator<<(ostream &os, const vector<T> &vec) {
    static constexpr const char *delim[] = {" ", ""};
    for (const auto &e : vec) os << e << delim[&e == &vec.back()];
    return os;
}

template <class Container>
struct Rev {
    Container &x_;
    inline Rev(Container &x) : x_(x) {}
    inline auto begin() { return rbegin(x_); }
    inline auto end() { return rend(x_); }
};
// }}} End Header

auto gridBFS(const vector<string> &grid, const vector<pair<int, int>> &starts,
             const string &walls)  // {{{
    -> vector<vector<int>> {
    static constexpr array<int, 4> dy{1, 0, -1, 0};
    static constexpr array<int, 4> dx{0, 1, 0, -1};
    static_assert(dy.size() == dx.size(), "Length of dy[], dx[] is different!");

    const int H = grid.size();
    const int W = grid[0].size();
    vector<vector<int>> dist(H, vector<int>(W, -1));
    queue<pair<int, int>> que;

    for (const auto &e : starts) {
        dist[e.first][e.second] = 0;
        que.emplace(e.first, e.second);
    }

    while (!que.empty()) {
        const int curY = que.front().first;
        const int curX = que.front().second;
        que.pop();
        for (int i = 0; i < dy.size(); ++i) {
            const int ny = curY + dy[i];
            const int nx = curX + dx[i];
            if (ny < 0 || nx < 0 || ny >= H || nx >= W) continue;
            if (dist[ny][nx] != -1) continue;
            if (walls.find(grid[ny][nx]) != string::npos) continue;
            dist[ny][nx] = dist[curY][curX] + 1;
            que.emplace(ny, nx);
        }
    }

    return dist;
}  // }}}

signed main() {
    ios_base::sync_with_stdio(false);
    cin.tie(nullptr);

    var(int, H, W);
    vector<string> mat(H);
    rep(i, 0, H) { read(mat[i]); }

    int ans = 0;
    rep(sy, 0, H) rep(sx, 0, W) {
        if (mat[sy][sx] == '#') continue;
        const auto dist = gridBFS(mat, {{sy, sx}}, "#");
        rep(gy, 0, H) rep(gx, 0, W) { chmax(ans, dist[gy][gx]); }
    }

    print(ans);

    return 0;
}


Measured memory_bytes: 256