/*

TASK:maze1

URL:http://train.usaco.org/usacoprob2?a=iHr5iXglQfJ&S=maze1

LANG:C++

*/

#include<cstdio>

#include<algorithm>

#include<cstring>

#include<queue>

#define N 305

using namespace std;

int n,m,dx[]={,,,-},dy[]={,,-,},dis[N][N],ans,vis[N][N];

char c[N][N];

struct node {

    int x,y,d;

};

queue<node>q;

void bfs(){

    for(int i=;i<=*m;i++)

    for(int j=;j<=*n;j++)

    if(!i||i==*m||!j||j==*n)

    if(!c[i][j]||c[i][j]==' '){

        memset(vis,,sizeof vis);

        q.push((node){i,j,});

        while(!q.empty()){

            node t=q.front();

            q.pop();

            int x=t.x,y=t.y,d=t.d;

            if(vis[x][y]||x<||y<||x>*m+||y>*n+||c[x][y]&&c[x][y]!=' ')continue;

            vis[x][y]=;

            dis[x][y]=min(dis[x][y],d);

            for(int i=;i<=;i++)

                    q.push((node){x+dx[i],y+dy[i],d+});

        }

    }

}

int main(){

    freopen("maze1.in","r",stdin);

    freopen("maze1.out","w",stdout);

    scanf("%d%d",&n,&m);

    for(int i=;i<=m*;i++)

        scanf("%*c%[^\n]",c[i]);

    memset(dis,0x3f3f3f3f,sizeof dis);

    bfs();

    for(int i=;i<=*m;i+=)

    for(int j=;j<=*n;j+=)

        ans=max(ans,dis[i][j]);

    printf("%d\n",ans+>>);

    return ;

}