#include<iostream>
#include<algorithm>
#include <iostream> 
#include <algorithm> 
#include<string>
#include<queue>
#include<utility>
#include<fstream>
#include<queue>
#include<set>
#include<stack>
#include<bits/stdc++.h>
#define ll long long int
#define ull unsigned long long int
#define I(a) scanf("%d",&a)
#define I2(a,b) scanf("%d%d",&a,&b)
#define I3(a,b,c) scanf("%d%d%d",&a,&b,&c)
#define L(a) scanf("%lld",&a)
#define L2(a,b) scanf("%lld%lld",&a,&b)
#define L3(a,b,c) scanf("%lld%lld%lld",&a,&b,&c)
#define PI(a) printf("%d",a)
#define PL(a) printf("%lld",a)
#define PT(t) printf("Case %d: ",t)
#define PB push_back
#define x first
#define y second
#define xx first.first
#define xy first.second
#define yx second.first
#define yy second.second
#define SC scanf
#define PC printf
#define NL printf("\n")
#define SET(a) memset(a,0,sizeof a)
#define SETR(a) memset(a,-1,sizeof a)
#define SZ(a) ((int)a.size())-1
#define f(i,a,b) for(int i=a;i<=b; i++)
#define fr(i,a,b) for(int i=a;i<=b; i++)
#define frr(i,a,b) for(int i=a;i>=b; i--)
#define frv(i,a)  for(int i=0;i<a.size();i++)
#define pi 2.0*acos(0.0)
#define R(a) freopen(a, "r", stdin);
#define W(a) freopen(a, "w", stdout);
#define CB(x) __builtin_popcount(x)
#define STN(a) stringtonumber<ll>(a)
#define lol printf("BUG\n")
#define Endl "\n"
#define mk make_pair
using namespace std;
template <class T> inline T BM(T p, T e, T M)
{
    ll ret = 1;
    for(; e > 0; e >>= 1)
    {
        if(e & 1) ret = (ret * p) % M;
        p = (p * p) % M;
    }
    return (T)ret;
}
template <class T> inline T gcd(T a, T b)
{
    if(b == 0)return a;
    return gcd(b, a % b);
}
template <class T> inline T mdINV(T a, T M)
{
    return BM(a, M - 2, M);
}
template <class T> inline T PW(T p, T e)
{
    ll ret = 1;
    for(; e > 0; e >>= 1)
    {
        if(e & 1) ret = (ret * p);
        p = (p * p);
    }
    return (T)ret;
}
 
 
template <class T>bool ISLEFT(T a, T b, T c)
{
    if(((a.xx - b.xx) * (b.yy - c.yy) - (b.xx - c.xx) * (a.yy - b.yy)) < 0.0)return 1;  //Uporer dike //A,b,c, x okkher ordera sorted
    else return 0;
}
 
 
typedef pair<int,int >P;
//////////////////////////
/////////////////////////
//#include<bits/stdc++.h>
using namespace std;
#define Base1   101LL
#define Base2   10000079LL
#define Base3   10000103LL
#define MOD1    1000000007LL
#define MOD2    1000000009LL
#define MOD3    1000000021LL
#define mx 100000//Number of charter of main string
#define mx1 25000// Number off character of total charcater
#define mx2 500// Number of pattern and number of ch of each pattern
 
 int dx[]= {1,-1,0,0};
 int dy[] = {0,0,1,-1};
 
int main(){

    int n,m;
    I2(n,m);
    char ar[n+2][m+2];
    for(int i =1;i<=n;i++){
        SC("%s", ar[i] +1);
    }
    int ans =0;

    for(int i=1;i<=n;i++){
        for(int j=1;j<=m;j++){
            if(ar[i][j] == '.'){

                int vis[n+2][m+2];
                memset(vis,-1,sizeof vis);
                queue<int>q;
                q.push(i);
                q.push(j);
                vis[i][j] = 0;
                while(!q.empty()){
                    int x = q.front();
                    q.pop();
                    int y = q.front();
                    q.pop();

                    for(int k=0;k<4;k++){
                        int x1 = x+ dx[k];
                        int y1 = y + dy[k];
                        if(x1>=1&&x1<=n&& y1>=1&&y1<=m && vis[x1][y1]==-1 &&ar[x1][y1] == '.'){
                            vis[x1][y1] = vis[x][y] +1;
                            //cout<<x<<" "<<y<<" "<<vis[x1][y1]<<endl;
                            ans = max(ans, vis[x1][y1]);
                            q.push(x1);
                            q.push(y1);
                        }
                    }
                    
                }
            }
        }
    }
    cout<<ans<<endl;
    return 0;
}

Measured memory_bytes: 256