#include <bits/stdc++.h>

#ifdef __LOCAL
    #define DBG(X) cout << #X << " = " << (X) << endl;
    #define SAY(X) cout << (X) << endl;
#else
    #define DBG(X)
    #define SAY(X)
#endif

#ifdef __LOCAL
    #include <filesystem>
    namespace fs = std::filesystem;
#endif

using namespace std;

using ll = long long int;
using ull = unsigned long long int;
using ld = long double;
using pii = pair<int, int>;
using pll = pair<ll, ll>;

int dx[]={1, 0, -1, 0, 0};
int dy[]={0, 1, 0, -1, 0};
const int INT_INF = (int)(2e9);
const ll  LL_INF = (ll)(2e18);
const int NIL = -1;
static mt19937 _g(time(nullptr));
 
std::string pad(int num){ char buffer[4]; std::snprintf(buffer, sizeof(buffer), "%03d", num); return buffer;}
inline ll randint(ll a, ll b) { ll w = (_g() << 31LL) ^ _g(); return a + w % (b - a + 1); }
inline void fast_io() { ios_base::sync_with_stdio(false); cin.tie(nullptr); cout.tie(nullptr); };
template<typename T> inline T sign(T x) { return T(x > 0) - T(x < 0); }
template<typename T, typename S> inline ostream& operator<<(ostream& os, const pair<T, S> p) { cout << "[" << p.first << ";" << p.second << "]"; return os; }
template<typename T> inline ostream& operator<<(ostream& os, const vector<T>& v) { for (auto el : v) cout << el << " "; return os; }
template<typename T> inline T fetch() { T ret; cin >> ret; return ret; }
template<typename T> inline vector<T> fetch_vec(int sz) { vector<T> ret(sz); for (auto& elem : ret) cin >> elem; return ret; }

int H,W;
vector<string> S;

void input(){
    fast_io();
    #ifdef __LOCAL
        fs::path p = __FILE__;
        fs::path input,output;
        input = output = p.parent_path();
        input += string("/input/") + string(p.stem()) + string(".txt");
        output += string("/output/") + string(p.stem()) + string(".txt");
        freopen(input.c_str(), "r", stdin);
        freopen(output.c_str(), "w", stdout);
    #endif
    cin >> H >> W;
    for (int i = 0; i < H; i++)
    {
        string s;
        cin >> s;
        S.push_back(s);
    }
}

bool isIn(pii n){
    return (0 <= n.first && n.first < H && 0 <= n.second && n.second < W); 
}

bool isWall(pii n){
    return (S[n.first][n.second]=='#');
}

int bfs(pii s, pii t){
    if(s==t) return 0;
    vector<vector<bool>> visited(H,vector<bool>(W,false));
    queue<pair<int,pii>> q;
    q.push({0,s});
    while(!q.empty()){
        int dist;
        pii now;
        tie(dist,now) = q.front();
        q.pop();
        for (int k = 0; k < 4; k++)
        {
            pii next;
            int nextDist = dist+1;
            next = now;
            next.first += dx[k];
            next.second += dy[k];
            if(next==t) return nextDist;
            if(!isIn(next)) continue;
            if(isWall(next)) continue;
            if(visited[next.first][next.second]) continue;
            q.push({nextDist,next});
            visited[next.first][next.second] = true;
        }
    }
    return 0;
}

void solve(){
    int ans=0;
    for (int i = 0; i < H; i++)
    {
        for (int j = 0; j < W; j++)
        {
            for (int k = 0; k < H; k++)
            {
                for (int l = 0; l < W; l++)
                {
                    pii s={i,j};
                    pii t={k,l};
                    if(s==t) continue;
                    if(isWall(s)||isWall(t)) continue;
                    ans = max(ans,bfs(s,t)); 
                }
                
            }
            
        }
        
    }
    cout << ans << endl;
}   

int main()
{
    input();
    solve();
    return 0;
}


Measured memory_bytes: 256