#include <bits/stdc++.h>
#define REP(i, n) for(int i = 0;i < n;i++)
#define REPR(i, n) for(int i = n;i >= 0;i--)
#define FOR(i, m, n) for(int i = m;i < n;i++)
#define FORR(i, m, n) for(int i = m;i >= n;i--)
#define SORT(v, n) sort(v, v+n);
#define VSORT(v) sort(v.begin(), v.end());
#define INF 999999999
#define M_PI 3.14159265358979323846

#ifdef __LOCAL
    #define DBG(X) cout << #X << " = " << (X) << endl;
    #define SAY(X) cout << (X) << endl;
#else
    #define DBG(X)
    #define SAY(X)
#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};
int dy[]={0, 1, 0, -1};
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;
int ans=0;
string Maze[21];

// (i,j) と (k,l)の距離
int dist(int const& i,int const& j,int const& k,int const& l){
    if(i==k and j==l) return 0;
    if(Maze[i][j]=='#') return 0;
    if(Maze[k][l]=='#') return 0;
    bool visited[21][21];
    visited[i][j]=true;

    stack<pii> container;
    map<pii,pii> toGoalPath;
    container.push(make_pair(i,j));

    while(!container.empty()){
        pii now = container.top();
        container.pop();
        for(int k=0;k<4;k++){
            int x,y;
            x = now.first+dx[k];
            y = now.second+dy[k];
            if(x<0|y<0|x>=H|y>=W)continue;
            if(visited[x][y]) continue;
            else visited[x][y]=true;
            if(x==k and y==l) {toGoalPath.emplace(make_pair(x,y),now); break;}
            if(Maze[x][y]=='#') continue;
            if(Maze[x][y]=='.') {container.push(make_pair(x,y)); toGoalPath.emplace(make_pair(x,y),now);}
        }

    }
    int ans=0;
    pii P = make_pair(k,l);
    while(P!=make_pair(i,j)){
        P = toGoalPath[P];
        ans++;
    }
    return ans;
}

int bfs(int const& i,int const& j){
    int ans=0;
    if(Maze[i][j]=='#') return 0;
    queue<pii> container;
    vector<vector<int> > Count;
    vector<vector<bool> > visited;
    Count.assign(H,vector<int>(W,0));
    visited.assign(H,vector<bool>(W,false));
    container.push(make_pair(i,j));

    while(!container.empty()){
        pii now = container.front();
        container.pop();
        visited[now.first][now.second]=true;

        for(int k=0;k<4;k++){
            int x,y;
            x = now.first+dx[k];
            y = now.second+dy[k];
            if(x<0|y<0|x>=H|y>=W) continue;
            if(Maze[x][y]=='#') continue;
            if(visited[x][y]) continue;
            if(Maze[x][y]=='.') {container.push(make_pair(x,y));visited[x][y]=true;Count[x][y] = Count[now.first][now.second]+1;}
        }

    }
    int maxi = 0;
    for (int i = 0; i < H; ++i) maxi = max(maxi, (int)*max_element(Count[i].begin(), Count[i].end()));

    return maxi;
}

int main()
{
    fast_io();
    #ifdef __LOCAL
        freopen("D_input.txt", "r", stdin);
        freopen("D_output.txt", "w", stdout);
    #endif

    cin >> H >> W;
    for(int i=0;i<H;i++){
        cin >> Maze[i];
    }

    // cout << (Maze[1][1]=='#') << endl;

    for(int i=0;i<H;i++)
        for(int j=0;j<W;j++)
            ans = max(ans,bfs(i,j));

    cout << ans << endl;
    return 0;
}


Measured memory_bytes: 256