#include <bits/stdc++.h>
#define rep(i, n) for (int i = 0; i < (n); ++i)
using namespace std;
typedef long long ll;
typedef pair<int, int> P;

const int INF = 1e6;
const int di = {-1,0,1,0};
const int dj = {0,-1,0,1};

int main()
{
    int h, w;
    cin >> h >> w;
    vector<string> s(h);
    rep(i,h) cin >> s[i];
    int ans = 0;
    rep(si,h)rep(sj,w) {
        if (s[si][sj] == '#') continue;
        vector<vector<int>> dish(h, vector<int>(w, INF));
        queue<P> q;
        auto update = [&](int i, int j, int x) {
            if (dish[i][j] != INF) return;
            dish[i][j] = x;
            q.push(P(i,j));
        };
        update(si,sj,0);
        while (!q.empty()) {
            int i = q.front().first;
            int j = q.front().second; q.pop();
            rep(dir,4) {
                int ni = i + di[dir], nj = j + dj[dir];
                if (ni < 0 || ni > h || nj < 0 || nj > w) continue;
                if (s[ni][nj] == '#') continue;
                update(ni,nj,dish[i][j] + 1);
            }
        }
        rep(i,h)rep(j,w) {
            if (dish[i][j] == INF) continue;
            ans = max(ans, dish[i][j]);
        }
    }
    cout << ans << endl;

    return 0;
}