#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 = 1000000007;
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; }

template<typename T>
vector<T> gen_v(size_t a) { return vector<T>(a); }
template<typename T, typename ...Ts>
auto gen_v(size_t a, Ts... ts) { return vector<decltype(gen_v<T>(ts...))>(a, gen_v<T>(ts...)); }
template<typename T, typename V>
typename enable_if<is_class<T>::value == 0>::type
fill_v(T &t, const V &v) { t = v; }
template<typename T, typename V>
typename enable_if<is_class<T>::value != 0>::type
fill_v(T &t, const V &v) { for (auto &e : t) fill_v(e, v); }

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 ? (e1.s == e2.s ? e1.d < e2.d : e1.s < e2.s) : 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 getEdges = [](const Graph &g) {
	Edges ret;
	for (auto &es : g)
		for (auto &e : es)
			ret.emplace_back(e);
	return ret;
};

auto dijkstra = [&](const Graph &g, int s, Array &dist) {
	int n = g.size();
	vector<bool> vis(n);
	vector<int> prev(n, -1);
	dist.assign(n, INF); dist[s] = 0;
	using State = tuple<Weight, int, int>;
	priority_queue<State, vector<State>, greater<State>> pq;
	pq.emplace(0, s, -1);
	while (pq.size()) {
		Weight d; int v, p; tie(d, v, p) = pq.top(); pq.pop();
		if (dist[v] < d)continue;
		vis[v] = true;
		prev[v] = p;
		for (auto &e : g[v]) {
			if (vis[e.d])continue;
			if (dist[e.d] > dist[v] + e.w) {
				dist[e.d] = dist[v] + e.w;
				pq.emplace(dist[e.d], e.d, v);
			}
		}
	}
	return prev;
};


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

	auto inrange = [&](int i, int j) { return i >= 0 && i < H && j >= 0 && j < W; };

	Graph g(H*W + 1);

	static const int di[] = { 1,0,-1,0 };
	static const int dj[] = { 0,1,0,-1 };

	int s = H * W;

	auto id = [&](int i, int j) {return i * W + j; };
	rep(i, 0, H)rep(j, 0, W) {
		if (v[i][j] == '#')addArc(g, s, id(i, j), 0);
		rep(d, 0, 4) {
			int ni = i + di[d], nj = j + dj[d];
			if (inrange(ni, nj) && v[ni][nj] == '.') {
				addArc(g, id(i, j), id(ni, nj), 1);
			}
		}
	}

	Array dist;
	dijkstra(g, s, dist);

	cout << *max_element(all(dist)) << endl;

	return 0;
}

Measured memory_bytes: 155304