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

///******************************** C o n t a i n e r ********************************///

typedef long long ll;
typedef unsigned long long ull;
typedef vector<int> vi;
typedef vector<ll> vl;
typedef pair<int,int> pii;
typedef pair<ll,ll> pll;
typedef vector<pii> vii;
typedef vector<pll> vll;

///*********************************** C o n s t ***********************************///

const int       N=1e6+123;
const double    PI = acos(-1);
const ll        MOD=1000000007;                 ///1e9+7
ll dx[] = {+1, 0, -1, 0, +1, +1, -1, -1};       ///first 4 for adjacent
ll dy[] = {0, +1, 0, -1, +1, -1, +1, -1};
ll dx8[]= {+1, +1, -1, -1, +2, +2, -2, -2};    ///knights move
ll dy8[]= {+2, -2, +2, -2, +1, -1, +1, -1};

///********************************** M a r c o ***********************************///

#define pb push_back
#define MP make_pair
#define F first
#define S second
#define test int tc; cin>>tc; while(tc--)
#define forn(i,n) for(i=0;i<n;i++)
#define all(a) a.begin(),a.end()
#define rall(a) a.rbegin(),a.rend()
#define sz(x) x.size()
#define el <<'\n'
#define sp <<' '
#define print(a) {for(auto x:a)cout<<x<<" ";cout<<endl;}
#define mem(a,b) memset(a, b, sizeof(a))
#define gcd(a,b) __gcd(a,b)
#define lcm(a,b) (a*(b/gcd(a,b)))
#define sqr(a) (a)*(a)

#define fastio() ios_base::sync_with_stdio(0);cin.tie(0);cout.tie(0);
#define file() freopen("input.txt","r",stdin);freopen("output.txt","w",stdout);
#define fraction() cout.unsetf(ios::floatfield); cout.precision(10); cout.setf(ios::fixed,ios::floatfield);

///*********************************** F u n c t i o n ***********************************///

ll powmod(ll a,ll b){ a%=MOD;if(!a) return 0;ll pr=1;while(b>0){if(b&1){pr*=a;pr%=MOD;--b;}a*=a;a%=MOD;b/=2;}return pr;}
ll modinverse(ll a){return powmod(a,MOD-2);}
bool isPrime(ll n){ if(n<=1)return false;if(n<=3)return true;if(n%2==0 or n%3==0)return false;for(ll i=5;i*i<=n;i+=6){if(n%i==0 or n%(i+2)==0)return false;}return true;}
void seive(bool a[]){ll mx=sqrt(N),ii,jj;for(ii=3;ii<=mx;ii+=2)if(!a[ii])for(jj=ii*ii;jj<N;jj+=2*ii)a[jj]=true;}
void numofdiv(ll a[]){ll mx=sqrt(N),ii,jj;for(ii=1;ii<=mx;ii++){for(jj=ii*ii;jj<N;jj+=ii){if(jj==ii*ii) a[jj]++; else a[jj]+=2;}}}
void sumofdiv(ll a[]){ll mx=sqrt(N),ii,jj;for(ii=1;ii<=mx;ii++){for(jj=ii*ii;jj<N;jj+=ii){if(jj==ii*ii) a[jj]+=ii; else a[jj]+=ii+jj/ii;}}}

///**************************************************** C o d e ****************************************************///
///-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=///
ll n,m,ans=0,lev[25][25];
string a[20];
bool vis[25][25];
bool val(ll x,ll y){return (x>=0 and x<n and y>=0 and y<m);}

void bfs(ll x,ll y)
{
    queue<pll> q;
    q.push({x,y});
    vis[x][y]=true;
    lev[x][y]=0;
    while(!q.empty()){
        x=q.front().F,y=q.front().S;
        q.pop();
        for(ll i=0;i<4;i++){
            ll r=x+dx[i],c=y+dy[i];
            if(val(r,c) and !vis[r][c] and a[r][c]=='.'){
                lev[r][c]=lev[x][y]+1;
                ans=max(ans,lev[r][c]);
                q.push({r,c});
                vis[r][c]=true;
            }
        }
    }

}

void _case()
{
    cin>>n>>m;
    for(ll i=0;i<n;i++) cin>>a[i];
    for(ll i=0;i<n;i++){
        for(ll j=0;j<m;j++){
            if(a[i][j]=='.'){
                mem(vis,false);
                bfs(i,j);
            }
        }
    }
    cout<<ans el;
}

main()
{
    fastio();
    _case();

}


///Thank you.


///Brainless_Loco Terminates Here!


Measured memory_bytes: 3672