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

#include <ext/pb_ds/assoc_container.hpp>
#include <ext/pb_ds/tree_policy.hpp>
#define ll long long int

using namespace __gnu_pbds;

typedef tree<pair<ll,pair<ll,ll> >, null_type, less<pair<ll,pair<ll,ll> > >, rb_tree_tag, tree_order_statistics_node_update>ordered_set;


#define me BOSS
#define bug() cout<<"AIkahne aise"<<endl
#define PI acos(-1)
#define mod 1000000007
/* Precode Start Here *\


power of 2 less than or equal to N

ll largest_power(ll N)
{
    //changing all right side bits to 1.
    N = N (N>>1);
    N = N| (N>>2);
    N = N| (N>>4);
    N = N| (N>>8);


//as now the number is 2 * x-1, where x is required answer, so adding 1 and dividing it by
 2.
            return (N+1)>>1;

}


     Sieve
void sieve()
{
    for(ll i=3; i<=2000; i+=2)
    {
        for(ll j=i*i; j<=4000000; j+=i)
            A[j]=1;
    }
    v.push_back(2);
    for(ll i=3; i<=4000000; i+=2)
        if(A[i]==0)
            v.push_back(i);
}

     Power Function Modulo P

ll modPow(ll a, ll x, ll p) {
    //calculates a^x mod p in logarithmic time.
    long res = 1;
    while(x > 0) {
        if( x % 2 != 0) {
            res = (res * a) % p;
        }
        a = (a * a) % p;
        x /= 2;
    }
    return res;
}


   factorial

ll fact(ll n)
{
    ll ans=1;
    for(ll i=1;i<=n;i++)
    {
        ans=(ans*i)%mod;
    }
    return ans;
}
     Calculating nCr using Fermats law Theorem

ll modInverse(ll a, ll p) {
    //calculates the modular multiplicative of a mod m.
    //(assuming p is prime).
    return modPow(a, p-2, p);
}


ll modBinomial(ll n, ll k, ll p) {
// calculates C(n,k) mod p (assuming p is prime).

    ll numerator = 1; // n * (n-1) * ... * (n-k+1)
    for (ll i=0; i<k; i++) {
        numerator = (numerator * (n-i) ) % p;
    }

    ll denominator = 1; // k!
    for (ll i=1; i<=k; i++) {
        denominator = (denominator * i) % p;
    }

    // numerator / denominator mod p.
    return ( numerator* modInverse(denominator,p) ) % p;
}
     num of divisor and euler totient

ll phi[1000001],mark[1000001],divi[1000001];;
void divisorPhi(ll n)
{
    for(int i=1;i<=n;i++)
        phi[i]=i;
    for(int i=2;i<=n;i++)
    {
        if(!mark[i])
        {
            for(int j=i;j<=n;j+=i)
            {
                mark[j]=1;
                phi[j]*=(1-1/(double)i);
            }
        }
    }
    for(ll i=1;i<=n;i++)
    {
        for(ll j=i;j<=n;j+=i)
            divi[j]++;
    }
}




/* Precode Ends */
const vector<pair<ll,ll>> offs = { { -1, 0 }, { 1, 0 }, { 0, -1 }, { 0, 1 } };
char A[31][31];
ll dp[501][501];
vector<ll>v;
map<ll,ll>myMap;
set<ll>mySet;
vector<pair<ll,ll> >vp;
#define id(p) p.first*m+p.second
void floyd_warshall(ll n)
{
    for(int k=0;k<n;k++)
    {
        for(int i=0;i<n;i++)
        {
            for(int j=0;j<n;j++)
            {
                dp[i][j]=min(dp[i][j],dp[i][k]+dp[k][j]);
            }
        }
    }
}
int main()
{
    ios::sync_with_stdio(false);
    cin.tie(nullptr);

    ll n,m;
    cin>>n>>m;
    for(int i=0; i<n; i++)
    {
        for(int j=0; j<m; j++)
        {
            cin>>A[i][j];
        }
    }
    for(int i=0; i<n*m; i++)
    {
        for(int j=0; j<n*m; j++)
            dp[i][j]=999999999;
    }
    for(int i=0; i<n; i++)
    {
        for(int j=0; j<m; j++)
        {
            if(A[i][j]=='#')
            {
                continue;
            }
            pair<ll,ll>source= {i,j};
            for(pair<ll,ll> off:offs)
            {
                ll u=i+off.first,v=j+off.second;
                if(u<0||v<0||u>=n||v>=m||A[u][v]=='#')
                    continue;
                dp[id(source)][id(make_pair(u,v))]=1;
            }
        }
    }
    floyd_warshall(n*m);
    ll ans=0;
    for(int i=0;i<n*m;i++)
    {
        for(int j=0;j<n*m;j++)
        {
            if(dp[i][j]!=999999999)
            {
                ans=max(ans,dp[i][j]);
            }
        }
    }
    cout<<ans<<endl;
    return 0;
}

