#include <bits/stdc++.h>

using namespace std;

const int OO = 1e9;
const double EPS = 1e-9;

#define ndl cout << '\n'
#define sz(v) int(v.size())
#define pb push_back
#define mp make_pair
#define fs first
#define sc second
#define present(a, x) (a.find(x) != a.end())
#ifdef LOCAL
#define db(...) ({cout << "> Line " << __LINE__  \
		<< ": "; _db(#__VA_ARGS__, __VA_ARGS__);})
#define RNG() rng()
#else
#define db(...) true
#define RNG() true
#endif

template<class T>
void _db(const char *dbStr, T e) {
	cout << dbStr << " = " << e << endl;
}
template<class T, class... L>
void _db(const char *dbStr, T e, L... r) {
  while(*dbStr != ',') cout << *dbStr++;
  cout << " = " << e << ','; _db(dbStr + 1, r...);
}
template<class S, class T>
ostream& operator<<(ostream& o, const map<S, T>& v) {
	o << "["; int i = 0;
	for (const pair<S, T>& pr : v) o << (!i++ ? "" : ", ") << "{"
	<< pr.fs << " : " << pr.sc << "}"; return o << "]";
}
template<template <class, class...> class S, class T, class... L>
ostream& operator<<(ostream& o, const S<T, L...>& v) {
	o << "["; int i = 0;
	for (const auto& e : v) o << (!i++ ? "" : ", ") << e;
	return o << "]";
}
template<class S, class T>
ostream& operator<<(ostream& o, const pair<S, T>& pr) {
	return o << "(" << pr.fs << ", " << pr.sc << ")";
}
ostream& operator<<(ostream& o, const string& s) {
	for (const char& c : s) o << c;
	return o;
}

template<class T> using V = vector<T>;
template<class T> using VV = V<V<T>>;
template<class T> using VVV = VV<V<T>>;
using ll = long long;
using pii = pair<int, int>;
using vi = V<int>;
using vii = V<pii>;
using vvi = VV<int>;
using mii = map<int, int>;
using umii = unordered_map<int, int>;
using si = set<int>;
using usi = unordered_set<int>;


int H, W;
int dr[] = {0, 0, 1, -1};
int dc[] = {1, -1, 0, 0};
char grid[25][25];

bool valid(int r, int c) {
	return r >= 0 && r < H && c >= 0 && c < W && grid[r][c] != '#';
}

int main() {
	#ifdef LOCAL
	auto stTime = clock();
//	freopen("in.txt", "r", stdin);
	mt19937_64 rng(chrono::steady_clock::now().time_since_epoch().count());
	#endif
	ios::sync_with_stdio(false);
	cout.precision(10);
	cin.tie(0);

	cin >> H >> W;

	for (int i = 0; i < H; ++i) {
		for (int j = 0; j < W; ++j) {
			cin >> grid[i][j];
		}
	}

	int ans = 0;

	for (int i = 0; i < H * W; ++i) {
		int r1 = i / W, c1 = i % W;

		db(r1, c1, grid[r1][c1]);

		if (grid[r1][c1] == '#')
			continue;

		// distance from (r1, c1) to other cells
		int dist[H][W];
		for (int x = 0; x < H; ++x) {
			for (int y = 0; y < W; ++y) {
				dist[x][y] = 1000;
			}
		}

		dist[r1][c1] = 0;
		queue<pii> q;
		q.push(mp(r1, c1));

		while (!q.empty()) {
			int r = q.front().fs, c = q.front().sc;
			q.pop();
			for (int k = 0; k < 4; ++k) {
				int to_r = r + dr[k], to_c = c + dc[k];
				if (!valid(to_r, to_c))
					continue;
				if (dist[to_r][to_c] == 1000)
					dist[to_r][to_c] = dist[r][c] + 1, q.push(mp(to_r, to_c));
			}
		}

		for (int x = 0; x < H; ++x) {
			for (int y = 0; y < W; ++y) {
				if (grid[x][y] == '.')
					ans = max(ans, dist[x][y]);
			}
		}
	}

	cout << ans << '\n';

	#ifdef LOCAL
	cout << "\n\n\nExecution time: " <<
		(clock() - stTime) * 1e3 / CLOCKS_PER_SEC << " ms" << endl;
	#endif
    return 0;
}


Measured memory_bytes: 256