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

# define int long long
int n, ca, sz[200005];
int h, w, hw[21][21];
int len = 0;
map<pair<pair<int, int>, pair<int, int>>, int>mpp;

signed main() {
	cin >> h >> w;
	string s;
	for (int i = 1; i <= h; i++) {
		cin >> s;
		for (int j = 0; j < w; j++) {
			if (s[j] == '.')hw[i][j + 1]  = 1;
			else hw[i][j + 1]  = -1;
		}
	}
	vector<pair<int, int>>stt, edd;
	for (int i = 1; i <= h; i++) {
		for (int j = 1; j <= w; j++) {
			if (hw[i][j] == 1) {
				stt.push_back({i, j});
				edd.push_back({i, j});
			}

		}
	}
//	cout<<stt.size()<<edd.size();

	int dx[4] = {-1, 0, 0, 1}, dy[4] = {0, -1, 1, 0};
	for (int i = 0; i < stt.size(); i++) {
		int dis[21][21], vis[21][21];
		memset(dis, 0, sizeof(dis));
		memset(vis, 0, sizeof(vis));
		queue<pair<pair<int, int>, int>>q;
		q.push({{stt[i].first, stt[i].second}, 0});
		vis[stt[i].first][stt[i].second] = 1;
		while (!q.empty()) {
			auto k = q.front();
			q.pop();

			//	cout<<k.first.first<<"  "<<k.first.second<<"  "<<k.second<<endl;

			for (int i = 0; i <= 3; i++) {
				int px = k.first.first + dx[i];
				int py = k.first.second + dy[i];
				if (px >= 1 && px <= h && py >= 1 && py <= w && !vis[px][py]) {
					if (hw[px][py] == 1) {
						dis[px][py] = max(	dis[px][py], k.second + 1);
						vis[px][py] = 1;
						q.push({{px, py}, k.second + 1});
						len = max(len, dis[px][py]);
					}

				}
			}

		}


	}
	cout << len;
}