#include<bits/stdc++.h>
using namespace std;
#define LL long long
#define MN 100005
int n,m;
LL sum;
int vis[25][25];
char ch[25][25];
const int fx[4]={1,-1,0,0},fy[4]={0,0,-1,1};
queue<int>Qx,Qy;
int bfs(int x,int y){
	memset(vis,0,sizeof(vis));
	vis[x][y]=1;
	Qx.push(x);Qy.push(y);
	int res=1;
//	cerr<<"==========="<<x<<" "<<y<<endl;
	while(!Qx.empty()){
		int x=Qx.front(),y=Qy.front();
//		cerr<<"? "<<x<<" "<<y<<endl;
		Qx.pop();Qy.pop();
		for(int i=0;i<4;++i){
			int X=x+fx[i],Y=y+fy[i];
			if(X<1||X>n||Y<1||Y>m)continue;
			if(ch[X][Y]=='#')continue;
			if(!vis[X][Y]){vis[X][Y]=vis[x][y]+1;
				res=max(res,vis[X][Y]);
				Qx.push(X);Qy.push(Y);
			}
		}
	}
	return res-1;
}
int main(){
	scanf("%d%d",&n,&m);
	for(int i=1;i<=n;++i){
		scanf("%s",ch[i]+1);
	}
	int ans=0;
	for(int i=1;i<=n;++i)
		for(int j=1;j<=m;++j){
			if(ch[i][j]!='#')ans=max(ans,bfs(i,j));
		}
	printf("%d",ans);
	return 0;
}
