#include <iostream>
#include <string>
#include <queue>
#include <stack>
#include <algorithm>
#include <list>
#include <vector>
#include <complex>
#include <utility>
#include <cstdio>
#include <cstdlib>
#include <cstring>
#include <cmath>
#include <climits>
#include <bitset>
#include <ctime>
#include <map>
#include <unordered_map>
#include <set>
#include <unordered_set>
#include <cassert>
#include <cstddef>
#include <iomanip>
#include <numeric>
#include <tuple>
#include <sstream>
#include <fstream>

#define REP(i,n) for(long (i)=0;(i)<(n);(i)++)
#define FOR(i,a,b) for(long (i)=(a);(i)<(b);(i)++)
#define RREP(i,a) for(long (i)=(a)-1;(i)>=0;(i)--)
#define FORR(i,a,b) for(long (i)=(a)-1;(i)>=(b);(i)--)
#define PI acos(-1.0)
#define DEBUG(C) cout<<C<<endl;
#define VI vector<int>
#define VII vector<VI>
#define VL vector<long>
#define VLL vector<VL>
#define VD vector<double>
#define VDD vector<VD>
#define PII pair<int,int>
#define PDD pair<double,double>
#define PLL pair<long,long>
#define ALL(a) (a).begin(),(a).end()
#define SORT(a) sort(ALL(a))
#define REVERSE(a) reverse(ALL(a))
#define MP make_pair
#define FORE(a,b) for(auto &&a:b)

using namespace std;
typedef long long LL;
typedef unsigned long long ULL;
const int INF=1e9;
const int MOD=INF+7;

VII dis;
vector<string> S;
int H,W;
int vh[]={0,0,-1,-1,-1,1,1,1};
int vw[]={1,-1,1,0,-1,1,0,-1};

void search(int h,int w){
    queue<PII> q;
    q.push(MP(h,w));
    while(!q.empty()){
        PII p=q.front(); q.pop();
        int ph=p.first,pw=p.second;
        REP(i,8){
            int dh=ph+vh[i],dw=pw+vw[i];
            if(dh>=H+2 || dh<0) continue;
            if(dw>=W+2 || dw<0) continue;
            if(S[dh][dw]=='.') continue;
            if(S[ph][pw]=='.'){
                dis[dh][dw]=1;
                q.push(MP(dh,dw));
            }else if(dis[dh][dw]==0 || dis[dh][dw]>dis[ph][pw]+1){
                dis[dh][dw]=dis[ph][pw]+1;
                q.push(MP(dh,dw));
            }
        }
    }
}

void out(){
    REP(i,dis.size()){
        REP(j,dis[i].size()){
            cout<<dis[i][j]<<" ";
        }
        cout<<endl;
    }
}

string str(char c,int n){
    string ret="";
    while(n--) ret+=c;
    return ret;
}

int main (void){
    cin>>H>>W;
    S=vector<string>(H+2);

    S.front()=str('.',H);
    S.back()=S.front();
    FOR(i,1,H+1) cin>>S[i];
    REP(i,S.size()){
        S[i]='.'+S[i]+'.';
    }

    dis=VII(H+2,VI(W+2,0));

    REP(i,H+2)REP(j,W+2){
        if(S[i][j]=='.'){
            search(i,j);
            dis[i][j]=-1;
        }
    }
    int ans=1;
    REP(i,H+2)REP(j,W+2){
        ans=max(ans,dis[i][j]);
    }
    //out();
    cout<<ans<<endl;
}