/*author: hyperion_1724
  date: 
*/

//Optional FAST

//#pragma GCC optimize("Ofast")
//#pragma GCC optimize("unroll-loops")
//#pragma GCC target("sse,sse2,sse3,ssse3,sse4,popcnt,fma,abm,mmx,avx,avx2,tune=native")

//Required Libraries

#include <bits/stdc++.h>
#include <ext/pb_ds/assoc_container.hpp>
#include <ext/pb_ds/tree_policy.hpp>
#include <ext/pb_ds/detail/standard_policies.hpp>

//Required namespaces

using namespace std;
using namespace __gnu_pbds;

//Required defines

#define endl '\n'

#define READ(X) cin>>X;
#define READV(X) long long X; cin>>X;
#define READAR(A,N) long long A[N]; for(long long i=0;i<N;i++) {cin>>A[i];}
#define rz(A,N) A.resize(N);
#define sz(X) (long long)(X.size())
#define pb push_back
#define pf push_front
#define fi first
#define se second
#define trav(x,A) for(auto &x:A)
#define FORI(a,b,c) for(long long a=b;a<c;a++)
#define FORD(a,b,c) for(long long a=b;a>c;a--)

//Required typedefs

typedef tree<long long,null_type,less<long long>,rb_tree_tag,tree_order_statistics_node_update> ordered_set;
typedef tree<long long,null_type,greater<long long>,rb_tree_tag,tree_order_statistics_node_update> ordered_set1;
typedef long long ll;
typedef long double ld;
typedef pair<int,int> pii;
typedef pair<long long,long long> pll;
typedef vector<int> veci;
typedef vector<long long> vecl;

//Required Constants

const long long inf=(long long)1e18;
const long long MOD=(long long)(1e9+7);
const long long INIT=(long long)(1e6+1);

//Required Functions

ll power(ll x,ll y) 
{ 
    if (y == 0) 
        return 1; 
    ll p = power(x, y/2) % MOD; 
    p = (p * p) % MOD; 
  
    return (y%2 == 0)? p : (x * p) % MOD; 
}
ll modInverse(ll a) 
{ 
   return power(a,MOD-2); 
   
}

//Work
ll H,W;
char A[20][20];
ll ans=0;
void Zeta(ll ux,ll uy)
{
    if (A[ux][uy]=='.')
    {
        ll depth[H][W];
        FORI(i,0,H)
        {
            FORI(j,0,W)
            {
                depth[i][j]=inf;
            }
        }
        deque<pll> bfs;
        bfs.pb({ux,uy});
        depth[ux][uy]=0;
        while (sz(bfs)>0)
        {
            pll f=bfs.front();
            ll i=f.fi;
            ll j=f.se;
            bfs.pop_front();
            if (i+1<H && A[i+1][j]=='.' && depth[i+1][j]==inf)
            {
                depth[i+1][j]=depth[i][j]+1;
                ans=max(ans,depth[i][j]+1);
                bfs.pb({i+1,j});
            }
            if (i-1>-1 && A[i-1][j]=='.' && depth[i-1][j]==inf)
            {
                depth[i-1][j]=depth[i][j]+1;
                ans=max(ans,depth[i][j]+1);
                bfs.pb({i-1,j});
            }
            if (j+1<W && A[i][j+1]=='.' && depth[i][j+1]==inf)
            {
                depth[i][j+1]=depth[i][j]+1;
                ans=max(ans,depth[i][j]+1);
                bfs.pb({i,j+1});
            }
            if (j-1>-1 && A[i][j-1]=='.' && depth[i][j-1]==inf)
            {
                depth[i][j-1]=depth[i][j]+1;
                ans=max(ans,depth[i][j]+1);
                bfs.pb({i,j-1});
            }
        }
    }
}
int main()
{
    
    ios::sync_with_stdio(false);
    cin.tie(0);
    cout.tie(0);

  
//-----------------------------------------------------------------------------------------------------------//

    READ(H);
    READ(W);
    FORI(i,0,H)
    {
        FORI(j,0,W)
        {
            READ(A[i][j]);
        }
    }
    FORI(i,0,H)
    {
        FORI(j,0,W)
        {
            Zeta(i,j);
        }
    }
    cout<<ans<<endl;

//-----------------------------------------------------------------------------------------------------------//


    
    return 0;
}

Measured memory_bytes: 256