#include<bits/stdc++.h>
using namespace std;

#define int long long
#define endl "\n"
#define REP(i,a,n) for(int i=a;i<n;++i)
#define REPR(i,a,n) for(int i=a;i>n;--i)
#define RUP(a,b) ((a+b-1)/(b))
#define ALL(v) (v).begin(),(v).end()
#define pb push_back
#define mp make_pair
#define mt make_tuple
//#define MOD 1000000007
#define INF LLONG_MAX/2

typedef long long ll;
typedef pair<int,int> Pii;
typedef tuple<int,int,int> Tiii;
typedef vector<int> Vi;
typedef vector<Vi> VVi;
typedef vector<Pii> VPii;
typedef vector<string> Vs;

template<class T> inline bool chmax(T& a,T b){if(a<b){a=b;return 1;}return 0;}
template<class T> inline bool chmin(T& a,T b){if(a>b){a=b;return 1;}return 0;}
template<class T> void YesNo(T a) {cout<<(a?"Yes":"No")<<endl;}
template<class T> void YESNO(T a) {cout<<(a?"YES":"NO")<<endl;}
void vin(Vi &v){REP(i,0,(v).size()) cin>>v[i];}
void vin(Vi &v,Vi &v2){REP(i,0,(v).size()) cin>>v[i]>>v2[i];}
void vout(Vi &v){for(int i=0;i<(v).size();i++) cout<<v[i]<<" ";cout<<endl;}
int gcd(int a,int b){return b?gcd(b,a%b):a;}
int lcm(int a,int b){return a/gcd(a,b)*b;}
void uniq(Vi &v){sort(v.begin(),v.end()); v.erase(unique(v.begin(),v.end()),v.end());}
int ctoi(char c){if(c>='0'&&c<='9'){return c-'0';}return 0;}
void accum(Vi &v){REP(i,1,(v).size()) v[i]+=v[i-1];}
bool comp(Pii a,Pii b){
    if(a.second != b.second) return a.second<b.second;
    else return a.first<b.first;
}



// Generated by 1.1.6 https://github.com/kyuridenamida/atcoder-tools  (tips: You use the default template now. You can remove this line by using your custom template)
// signed main(){
//     const Vi dx = {0,1,0,-1};
//     const Vi dy = {1,0,-1,0};

//     int h,w;
//     cin>>h>>w;
//     Vs s(h);
//     REP(i,0,h) cin>>s[i];

//     REP(I,0,h){
//         REP(J,0,w){
//             int s=I,t=J;

//     VVi used(h,Vi(w,0));
//     queue<Tiii> q;
//     q.push(mt(s,t,0));
//     while(!q.empty()){
//         int nowx,nowy,dnow;
//         tie(nowy,nowx,dnow)=q.front();
//         q.pop();
//         if(used[nowy][nowx]) continue;
//         used[nowy][nowx]=dnow;
//         if(now==)
//         REP(d,0,dx.size()){
//             int newx=nowx+dx[d],newy=nowy+dy[d];
//             if(0<=newx && newx<w && 0<=newy && newy<h){
//                 if(s[newy][newx]=='.' && used[newy][newx]==0) q.push(mt(newy,newx,dnow+1));
//             }
//         }
//     }
//     REP(i,0,h) REP(j,0,w){chmax(tans,used[i][j]}

//         }
//     }


//    return 0;
// }

signed main(){
    const Vi dx = {0,1,0,-1};
    const Vi dy = {1,0,-1,0};

    int h,w,ans=0;
    cin>>h>>w;
    Vs s(h);
    REP(i,0,h) cin>>s[i];

    REP(si,0,h){
        REP(sj,0,w){
            REP(gi,0,h){
                REP(gj,0,w){

if(s[si][sj]=='#' || s[gi][gj]=='#') continue;
VVi used(h,Vi(w,0));
queue<Tiii> q;
q.push(mt(si,sj,0));
while(!q.empty()){
    int nowx,nowy,dnow;
    tie(nowy,nowx,dnow)=q.front();
    q.pop();
    if(used[nowy][nowx]) continue;
    used[nowy][nowx]=dnow;
    if(nowy==gi && nowx==gj) break;
    REP(d,0,dx.size()){
        int newx=nowx+dx[d],newy=nowy+dy[d];
        if(0<=newx && newx<w && 0<=newy && newy<h){
            if(s[newy][newx]=='.') q.push(mt(newy,newx,dnow+1));
        }
    }
}
//cout<<si<<" "<<sj<<" "<<gi<<" "<<gj<<" "<<used[gi][gj];
chmax(ans,used[gi][gj]);

                }
            }
        }
    }
    cout<<ans<<endl;
}

Measured memory_bytes: 256