THMECUNG

#include <bits/stdc++.h>
using namespace std;

using ll = long long;
using ii = pair<int, int>;

const int maxN = 1e3 + 5;
const int maxM = 1e4 + 5;
const ll INF = 2e18 + 7;
const int MOD = 998244353;

int dx[4] = {0, 1, 0, -1};
int dy[4] = {1, 0, -1, 0};

int n, m;
string s[maxM];

int ans = 0;
int h[maxN][maxN];
bool visited[maxN][maxN], visit[maxN][maxN];

void bfs(int x, int y) {
    memset(visit, false, sizeof(visit));
    visit[x][y] = true;
    memset(h, 0, sizeof(h));
    queue<ii> q;
    q.push(make_pair(x, y));

    while (!q.empty()) {
        int ux = q.front().first;
        int uy = q.front().second;
        q.pop();

        for (int i = 0; i < 4; i++) {
            int u = ux + dx[i];
            int v = uy + dy[i];

            if (u <= 0 || u > n) continue;
            if (v <= 0 || v > m) continue;
            if (visit[u][v]) continue;

            if (s[u][v] == '.') {
                visit[u][v] = true;
                q.push(make_pair(u, v));
                h[u][v] = h[ux][uy] + 1;
                ans = max(ans, h[u][v]);
            }
        }
    }
}

int main() {
#ifdef LOCAL
    freopen("in4.txt", "r", stdin);
#else
    freopen("THMECUNG.inp", "r", stdin);
    freopen("THMECUNG.out", "w", stdout);
#endif
    ios_base::sync_with_stdio(false);
    cin.tie(nullptr);

    cin >> n >> m;
    for (int i = 1; i <= n; i++) {
        cin >> s[i];
        s[i] = ' ' + s[i];
    }

    for (int i = 1; i <= n; i++)
        for (int j = 1; j <= m; j++)
            if (s[i][j] == '.'  && !visited[i][j]) {
                visited[i][j] = true;
                bfs(i, j);
            }

    cout << ans << '\n';

    return 0;
}