#include <bits/stdc++.h>
using namespace std;
#ifdef LOCAL
#include "debug.h"
#else
#define dump(...)
#endif
#define endl "\n"
#define ll long long
#define FOR(i, a, b) for (int i = (int)(a); i < (int)(b); i++)
#define REP(i, x) for (int i = 0; i < (int)(x); i++)
#define REPS(i, x) for (int i = 1; i <= (int)(x); i++)
#define RREP(i, x) for (int i = ((int)(x)-1); i >= 0; i--)
#define RREPS(i, x) for (int i = ((int)(x)); i > 0; i--)
#define INF (1 << 30)
#define LLINF (1LL << 60)
#define vi vector<int>
#define vvi vector<vector<int>>
#define pi pair<int, int>
#define ALL(a) (a).begin(), (a).end()
#define BIT(n) (1LL << (n))
#define UNIQUE(v) v.erase(unique(v.begin(), v.end()), v.end()) //sortしてからつかうこと
constexpr ll MOD = 1e9 + 7;
int dx[8] = {1, 0, -1, 0, 1, 1, -1, -1};
int dy[8] = {0, 1, 0, -1, 1, -1, 1, -1};
ll A, B, C, D, E, F, G, H, N, M, L, K, P, Q, R, W, X, Y, Z;
string S, T;
ll ans = 0;

template <typename T>
istream &operator>>(istream &is, vector<T> &vec) {
    for (T &x : vec)
        is >> x;
    return is;
}

//0-indexedに注意
class Graph {
    struct edge {
        int from, to, cost;
        edge(int from, int to, int cost) : from(from), to(to), cost(cost) {}
    };

public:
    int verN;
    int edgeN = 0;
    vector<edge> edge_set;
    vector<vector<pair<int, int>>> adj_list;
    Graph(int verN) : verN(verN), adj_list(verN) {}

    void add_directed_edge(int from, int to, int cost = 1) {
        edge_set.push_back(edge(from, to, cost));
        adj_list[from].push_back(make_pair(to, cost));
        edgeN++;
    }

    void add_undirected_edge(int from, int to, int cost = 1) {
        edge_set.push_back(edge(from, to, cost));
        edge_set.push_back(edge(to, from, cost));
        adj_list[from].push_back(make_pair(to, cost));
        adj_list[to].push_back(make_pair(from, cost));
        edgeN += 2;
    }

    bool bellmanford(int start, vector<int> &d) {
        d.assign(verN, INF);
        for (int i = 0; i < verN; i++) {
            d[i] = INF;
        }
        d[start] = 0;
        for (int i = 0; i < verN; i++) {
            for (int j = 0; j < edgeN; j++) {
                edge e = edge_set[j];
                if (d[e.from] != INF && d[e.to] > d[e.from] + e.cost) {
                    d[e.to] = d[e.from] + e.cost;
                    if (i == verN - 1) {
                        return false;
                    }
                }
            }
        }
        return true;
    }

    vector<int> dijkstra(int start) {
        vector<int> d(verN, INF);
        d[start] = 0;
        priority_queue<pair<int, int>, vector<pair<int, int>>, greater<pair<int, int>>> pq;
        vector<int> used(verN, 0);
        pq.push(make_pair(0, start));
        while (!pq.empty()) {
            int now = pq.top().second;
            pq.pop();
            used[now] = 1;
            for (auto adj_pair : adj_list[now]) {
                int adj = adj_pair.first;
                int dist = adj_pair.second;
                if (used[adj])
                    continue;
                if (d[adj] > d[now] + dist) {
                    d[adj] = d[now] + dist;
                    pq.push(make_pair(d[adj], adj));
                }
            }
        }
        return move(d);
    }

    vector<vector<int>> warshallfloyd() {
        vector<vector<int>> d(verN, vector<int>(verN, INF - 1));
        for (int i = 0; i < verN; i++) {
            for (auto adj_pair : adj_list[i]) {
                int adj = adj_pair.first;
                int dist = adj_pair.second;
                d[i][adj] = dist;
            }
        }
        for (int i = 0; i < verN; i++) {
            d[i][i] = 0;
        }
        for (int k = 0; k < verN; k++) {         // 経由する頂点
            for (int i = 0; i < verN; i++) {     // 始点
                for (int j = 0; j < verN; j++) { // 終点
                    d[i][j] = min(d[i][j], d[i][k] + d[k][j]);
                }
            }
        }
        for (int i = 0; i < verN; i++) {
            for (int j = 0; j < verN; j++) {
                if (d[i][j] == INF - 1) {
                    d[i][j] = -1;
                }
            }
        }
        return move(d);
    }
};

signed main() {
    cin >> H >> W;
    vector<vector<char>> s(H + 1, vector<char>(W + 1, '#'));
    Graph g(H * W);
    REP(h, H)
    REP(w, W) {
        cin >> s[h][w];
    }
    REP(h, H)
    REP(w, W) {
        if (s[h][w] == '#') {
            continue;
        }
        if (s[h + 1][w] == '.') {
            g.add_undirected_edge(h * W + w, (h + 1) * W + w);
        }
        if (s[h][w + 1] == '.') {
            g.add_undirected_edge(h * W + w, h * W + (w + 1));
        }
    }
    auto d = g.warshallfloyd();
    REP(i, H * W) {
        ans = max(int(ans), *max_element(ALL(d[i])));
        dump(d[i]);
    }
    cout << ans << endl;
}

Measured memory_bytes: 896