#include "bits/stdc++.h"
using namespace std;
#ifdef _DEBUG
#include "dump.hpp"
#else
#define dump(...)
#endif

//#define int long long
#define rep(i,a,b) for(int i=(a);i<(b);i++)
#define rrep(i,a,b) for(int i=(b)-1;i>=(a);i--)
#define all(c) begin(c),end(c)
const int INF = sizeof(int) == sizeof(long long) ? 0x3f3f3f3f3f3f3f3fLL : 0x3f3f3f3f;
const int MOD = 1'000'000'007;
template<class T> bool chmax(T &a, const T &b) { if (a < b) { a = b; return true; } return false; }
template<class T> bool chmin(T &a, const T &b) { if (b < a) { a = b; return true; } return false; }

using Weight = int;
struct Edge {
	int s, d; Weight w;
	Edge() {};
	Edge(int s, int d, Weight w) : s(s), d(d), w(w) {};
};
bool operator<(const Edge &e1, const Edge &e2) { return e1.w < e2.w; }
bool operator>(const Edge &e1, const Edge &e2) { return e2 < e1; }
inline ostream &operator<<(ostream &os, const Edge &e) { return (os << '(' << e.s << ", " << e.d << ", " << e.w << ')'); }

using Edges = vector<Edge>;
using Graph = vector<Edges>;
using Array = vector<Weight>;
using Matrix = vector<Array>;

void addArc(Graph &g, int s, int d, Weight w = 1) {
	g[s].emplace_back(s, d, w);
}
void addEdge(Graph &g, int a, int b, Weight w = 1) {
	addArc(g, a, b, w);
	addArc(g, b, a, w);
}

auto zeroOneBfs = [&](const Graph &g, int s, Array &dist) {
	int n = g.size();
	deque<int> dq = { s };
	dist.assign(n, INF);
	dist[s] = 0;
	while (dq.size()) {
		int v = dq.front(); dq.pop_front();
		for (auto &e : g[v]) {
			if (dist[e.d] == INF) {
				dist[e.d] = dist[v] + e.w;
				if (e.w == 0)
					dq.push_front(e.d);
				else
					dq.push_back(e.d);
			}
		}
	}
};

signed main() {
	cin.tie(0);
	ios::sync_with_stdio(false);
	int W, H; cin >> W >> H;
	vector<vector<char>> C(H, vector<char>(W));
	rep(i, 0, H) rep(j, 0, W) { cin >> C[i][j]; }

	Graph g(H*W);

	static const int di[] = { 1,0,-1,0 };
	static const int dj[] = { 0,1,0,-1 };
	rep(i, 1, H - 1)rep(j, 1, W - 1) {
		rep(d, 0, 4) {
			int ni = i + di[d], nj = j + dj[d];
			addArc(g, i*W + j, ni*W + nj, C[ni][nj] == '#');
		}
	}

	rep(i, 0, H)rep(j, 0, W) {
		if (C[i][j] == '.') {
			Array dist;
			zeroOneBfs(g, i*W + j, dist);
			int ans = 0;
			rep(ni, 0, H)rep(nj, 0, W) {
				if (C[ni][nj] == '.')
					chmax(ans, dist[ni*W + nj]);
			}
			cout << ans << endl;
			return 0;
		}
	}

	return 0;
}