#include <ext/pb_ds/assoc_container.hpp> 
#include <ext/pb_ds/tree_policy.hpp>  
#include <functional>
#include <iostream> 
#include<bits/stdc++.h>
using namespace __gnu_pbds;  
using namespace std;  
// Declaring ordered_set 
typedef tree<long long int, null_type, less<long long int>, rb_tree_tag,  
            tree_order_statistics_node_update>  
    ordered_set;  
typedef pair<long long int,long long int> pa;
#define resize(A) A.resize(unique(A.begin(), A.end()) - A.begin())
#define pb push_back
#define  MAX 1000005
long long int MOD=998244353;
long long int INF=1e18+4;

///////////////////////////////////////////////sieve////////////////////////////////////////////////////////////////////
 /* bool B[MAX];
vector<long long int>V1;
 void sieve()
 {
    long long int i,j;
    for(i=2;i<MAX;i++)
    {
      if(!B[i])
     { 
          V1.pb(i);
        for(j=2*i;j<MAX;j=j+i)
             B[j]=true;
      }
    }
 }
 */

 
 
//////////////////////////////////////////////////////////D-S-U/////////////////////////////////////////////////////////
/*long long int parent[MAX];
long long int size[MAX];
void initialise()
{ //cout<<MAX<<" ";
    for(long  long int i=0;i<MAX;i++)
    {
        parent[i]=i;
        //cout<<i<<" ";
        size[i]=1;
    }
}
long long int  root(long long int x)
{
    while(x!=parent[x])
    {
        parent[x]=parent[parent[x]];
        x=parent[x];
    }
    return x;
}
 
    
void union1(long long int x,long long int y)
{
    if(size[x]>=size[y])
    {
        size[x]+=size[y];
        parent[y]=x;
        
    }
    else
    {
        size[y]+=size[x];
        parent[x]=y;
    }
}*/
//////////////////////////////////////////////////////MODPOWER//////////////////////////////////////////////////////////

 
/*long long int modulo(long long int a,long long  int b, long long int n){
    long long int  x=1, y=a%n; 
    while (b > 0) {
        if (b%2 == 1) {
            x = (x*y) % n; // multiplying with base
        }
        y = (y*y) % n; // squaring the base
        b /= 2;
    }
    return x % n;
}*/
 
 
 
////////////////////////////////////////////////BIT/////////////////////////////////////////////////////////////////////
/*long long int sum(long long int bit[],long long int in,long long int n)
{
    //in=in+1;
    long long int sum=0;
    if(in==0)
        return 0;
    while(in>0)
    {
      sum=sum+bit[in];
        
        in=in-(in&(-in));
    }
    return sum;
}
void update(long long int bit[],long long int in,long long int n,long long int val)
{
    // in=in+1;
    
    while(in<=n)
    {
       // cout<<in<<" ";
        bit[in]+=val;
        in=in+ (in &(-in));
    }
}*/
long long int R[4]={1,-1,0,0};
long long int C[4]={0,0,1,-1};
int main()
{
ios_base::sync_with_stdio(false); 
cin.tie(NULL);
cout.tie(NULL);
long long int d,i,w,j,k=0,l=0,r=0,m=0,n,c,p=0,cnt=0,cnt1=0,x,y,z=0,a,b,low,mid,high,ans=0,maxi=0,mini=INF,t1,x1,y1,t,sum=0,i1,j1;
string sl,s1;    long long qt,xx;        char jj;
    
    cin>>n>>m;
    string s[n];
    long long int dist[n][m];
    queue<pa>q;
    maxi=0;
    for(i=0;i<n;i++)
    {
        cin>>s[i];
    }
    for(i=0;i<n;i++)
    {
        for(j=0;j<m;j++)
        {
            if(s[i][j]=='#')
                continue;
            for(i1=0;i1<n;i1++)
            {
                for(j1=0;j1<m;j1++)
                {
                    dist[i1][j1]=-1;
                }
            }
            q.push({i,j});
            dist[i][j]=0;
            while(!q.empty())
            {
                a=q.front().first;
                b=q.front().second;
                q.pop();
                for(k=0;k<4;k++)
                {
                    i1=R[k]+a;
                    j1=C[k]+b;
                    if(i1>=0 && i1<n && j1>=0 && j1<m && s[i1][j1]=='.' && dist[i1][j1]==-1 )
                    {
                        dist[i1][j1]=1+dist[a][b];
                        q.push({i1,j1});
                    }
                }
            }
                 for(i1=0;i1<n;i1++)
            {
                for(j1=0;j1<m;j1++)
                {
                    maxi=max(maxi,dist[i1][j1]);
                }
            }
            
        }
    }
    cout<<maxi<<endl;
 
}
    

    
    
    


Measured memory_bytes: 256