#define HAVE_STRUCT_TIMESPEC
#include<bits/stdc++.h>
using namespace std;
int n,m,ans;
char s[25][25];
bool vis[25][25];
struct node{
	int x,y,dis;
};
int direction[4][2]={{1,0},{0,1},{-1,0},{0,-1}};
void bfs(int x,int y){
	if(s[x][y]=='#')
        return ;
	memset(vis,0,sizeof(vis));
	queue<node>q;
	q.push((node){x,y,0});
	vis[x][y]=1;
	while(!q.empty()){
		node now=q.front();
		q.pop();
		ans=max(ans,now.dis);
		for(int i=0;i<4;++i){
			int temp_x=now.x+direction[i][0],temp_y=now.y+direction[i][1];
			if(temp_x<=0||temp_x>n||temp_y<=0||temp_y>m||s[temp_x][temp_y]=='#'||vis[temp_x][temp_y])
                continue;
			q.push((node){temp_x,temp_y,now.dis+1});
			vis[temp_x][temp_y]=1;
		}
	}
}
int main(){
    ios::sync_with_stdio(false);
    cin.tie(NULL);
    cout.tie(NULL);
	cin>>n>>m;
	for(int i=1;i<=n;++i)
        cin>>s[i]+1;
	for(int i=1;i<=n;++i)
        for(int j=1;j<=m;++j)
            bfs(i,j);
	cout<<ans;
	return 0;
}
