/*
Author : N_o_o_B
Created : January 12 2020 17:39:02 
*/
#include <bits/stdc++.h>
using namespace std;
/*
#include <ext/pb_ds/assoc_container.hpp>
#include <ext/pb_ds/tree_policy.hpp>
using namespace __gnu_pbds;
*/
#define TRACE
#ifdef TRACE
#define trace(...) { cerr<<"[ "; __trace__(#__VA_ARGS__, __VA_ARGS__);}
#undef endl
template <typename Arg1,typename Arg2>
ostream& operator << (ostream& out, const pair<Arg1,Arg2> &x) {
    return out<<"("<<x.first<<","<<x.second<<")";
}
 
template <typename Arg1>
ostream& operator << (ostream& out, const vector<Arg1> &a) {
    out<<"[";for(const auto &x:a)out<<x<<",";return out<<"]";
}
 
template <typename Arg1>
ostream& operator << (ostream& out, const set<Arg1> &a) {
    out<<"[";for(const auto &x:a)out<<x<<",";return out<<"]";
}
 
template <typename Arg1,typename Arg2>
ostream& operator << (ostream& out, const map<Arg1,Arg2> &a) {
    out<<"[";for(const auto &x:a)out<<x<<",";return out<<"]";
}
 
template <typename Arg1>
void __trace__(const string name, Arg1&& arg1){
    cerr << name << " : " << arg1 << " ] " << endl;
}
 
template <typename Arg1, typename... Args>
void __trace__(const string names, Arg1&& arg1, Args&&... args){
    const string name = names.substr(0,names.find(','));
    cerr<<name<<" : "<<arg1<<" | ";
    __trace__(names.substr(1+(int)name.size()), args...);
}
#else
    #define trace(args...)
#endif
 
typedef long long ll;
typedef long double ld;
typedef pair<int,int> pii;
typedef vector<int> vi;
typedef vector<pair<int,int>> vii;
typedef vector<vi> vvi;
typedef vector<ll> vl;
typedef vector<pair<ll,ll>> vll;
typedef vector<vl> vvl;
//typedef tree<pii, null_type, less<pii>, rb_tree_tag, tree_order_statistics_node_update> oset;
#define fori(i,n) for(int i=0;i<n;i++)
#define rep(i,a,b) for(int i=a;i<=b;i++)
#define repd(i,a,b) for(int i=a;i>=b;i--)
#define ford(i,n)  for(int i=n-1;i>=0;i--)
#define trav(x,a) for(auto& x:a)
#define all(x) x.begin(),x.end()
#define pb push_back
#define endl '\n'
#define sz(a) (int)a.size()
#define fi first
#define se second
clock_t time_p=clock();
void time_taken()
{
    time_p=clock()-time_p;
    cerr<<"Time Taken : "<<(float)(time_p)/CLOCKS_PER_SEC<<"\n";
}
//const ll mod=998244353;
const ll mod=1e9+7;
const ll INF=1e18;
vector<vector<char>> a;
int n,m;
int dx[4] = {-1,+1,0,0};
int dy[4] = {0,0,+1,-1};
int f(pii st,pii en){
    vvi dis(n,vi(m,-1));
    queue<pii> q;
    q.push(st);
    dis[st.fi][st.se] = 0;
    while(sz(q)){
        int x = q.front().fi;
        int y = q.front().se;
        if(make_pair(x,y) == en) return dis[x][y];
        q.pop();
        fori(i,4){
            int tox = x + dx[i];
            int toy = y + dy[i];
            if(tox >= 0 and tox < n and toy >= 0 and toy < m and dis[tox][toy]==-1 and a[tox][toy]=='.'){
                q.push({tox,toy});
                dis[tox][toy] = dis[x][y] + 1;
            }
        } 
    }
}
int main(){
    ios_base::sync_with_stdio(false);
    cin.tie(NULL);cout.tie(NULL);
    cout.precision(12);cout << fixed;


    cin >> n >> m;
    a.resize(n);
    fori(i,n){
        a[i].resize(m);
    }
    vii roads;
    fori(i,n){
        fori(j,m){
            cin >> a[i][j];
            if(a[i][j]=='.'){
                roads.pb({i,j});
            }
        }
    }
    //trace(a);
    int ans = 0;
    fori(i,sz(roads)){
        for(int j=i+1;j<sz(roads);j++){
            ans = max(ans,f(roads[i],roads[j]));
        }
    }
    cout << ans << endl;
    time_taken();
    return 0;
}

Measured memory_bytes: 384