#include <bits/stdc++.h>
using namespace std;
#define inf (int)(1e9)

int main(){
	int H, W, i, j, ans = 0;
	scanf("%d%d", &H, &W);
	vector<string> S(H);
	for(i = 0; i < H; i++){
		cin >> S[i];
	}
	int dx[4] = {0, 1, 0, -1};
	int dy[4] = {1, 0, -1, 0};
	for(i = 0; i < H; i++){
		for(j = 0; j < W; j++){
			if(S[i][j] == '#'){
				continue;
			}
			vector<vector<int>> dis(H, vector<int>(W, inf));
			queue<pair<pair<int, int>, int>> q;
			q.push(make_pair(make_pair(i, j), 0));
			while(q.size() > 0){
				int x = q.front().first.first;
				int y = q.front().first.second;
				int v = q.front().second;
				q.pop();
				if(x < 0 || H <= x || y < 0 || W <= y || S[x][y] == '#' || dis[x][y] <= v){
					continue;
				}
				dis[x][y] = v;
				ans = max(ans, v);
				for(int d = 0; d < 4; d++){
					q.push(make_pair(make_pair(x + dx[d], y + dy[d]), v + 1));
				}
			}
		}
	}
	printf("%d\n", ans);
	return 0;
}