#include<bits/stdc++.h>
#include<ext/pb_ds/assoc_container.hpp>
//#pragma comment(linker, "/STACK:5000000000")
//#pragma GCC optimize "-O3"
//#pragma GCC optimize("Ofast,no-stack-protector")
//#pragma GCC target("mmx,avx,fma")
//#pragma GCC optimize ("unroll-loops")

using namespace __gnu_pbds;
using namespace std;

template <class T>
istream& operator >> (istream& in, vector<T>& v){ for(auto& e : v){ in >> e;} return in;}
template <class T>
istream& operator >> (istream& in, vector<pair<T, T> >& v){ for(auto& e : v){ in >> e.first >> e.second;} return in;}

#define int long long
//#define int short int
#define pb push_back
#define mk make_pair
#define X first
#define Y second
#define cont continue
#define ret return
#define For(i, a, b) for (int i = a; i < b;i++)
#define forn(i, a) for (int i = 0;i < a;i++)
#define ford(i, a, b) for (int i = b - 1;i >= a;i--)
#define fordn(i, a) for (int i = a - 1;i >= 0;i--)
#define forr(x, arr) for(auto &x: arr)
#define all(x) x.begin(),x.end()
#define rall(x) x.rbegin(),x.rend()
#define REE(s_) {cout << s_ << '\n';exit(0);}
#define makeunique(x) sort(all(x)), (x).resize(unique(all(x)) - (x).begin())
#ifdef debug
    template <char d = ' ', class T>
    ostream& operator << (ostream& out, vector<T>& v){ out << "[";for(int i = 0;i < v.size();i++){out << v[i];if (i != v.size() - 1) out << ", ";} out << "]"; return out; }
    template <char d = ' ', class T>
    ostream& operator << (ostream& out, vector<pair<T, T> >& v){ out << "[";for(int i = 0;i < v.size();i++){ out << ' ' << v[i].first << ", " << v[i].second; if (i != v.size() - 1) out << '\n'; } out << "]"; return out;}
    template <char d = ' ', class T>
    ostream& operator << (ostream& out, set<T>& v){out << "["; for(auto i = v.begin();i != v.end();i++){ out << *i; if (*i != *v.rbegin()) out << ", ";} out << "]"; return out;}
    template <char d = ' ', class T>
    ostream& operator << (ostream& out, multiset<T>& v){out << "["; for(auto i = v.begin();i != v.end();i++){ out << *i; if (*i != *v.rbegin()) out << ", ";} out << "]"; return out;}
#else
template <char d = ' ', class T>
ostream& operator << (ostream& out, vector<T>& v){ for(auto& e : v){ out << e << d;} return out;}
template <char d = ' ', class T>
ostream& operator << (ostream& out, vector<pair<T, T> >& v){ for(auto& e : v){ out << e.first << ' ' << e.second << '\n';} return out;}
template <char d = ' ', class T>
ostream& operator << (ostream& out, set<T>& v){ for(auto& e : v){ out << e << d;} return out;}
#endif
typedef long long ll;
typedef long double ldd;
typedef pair<int, int> pii;
typedef vector<int> vi;
typedef vector<string> vs;
typedef vector<pii> vpi;
typedef vector<vi> vvi;
typedef map<int, int> mii;
typedef unordered_map<string, int> msi;
typedef set<int> si;
typedef tree<int,null_type,less<int>,rb_tree_tag,tree_order_statistics_node_update> indexed_set;
template<class T> bool uin(T &a, T b) { return a > b ? (a = b, true) : false; }
template<class T> bool uax(T &a, T b) { return a < b ? (a = b, true) : false; }
template <typename T> string toString(T val){ostringstream oss; oss << val; return oss.str();}
template<typename T> T sqr(T a) {return a * a;}
mt19937 gen(chrono::high_resolution_clock::now().time_since_epoch().count());
const int dx[4] = {0, 1, 0, -1};
const int dy[4] = {1, 0, -1, 0};
//const int dx[8] = {0, 0, 1, 1, 1, -1, -1, -1};
//const int dy[8] = {1, -1, 1, 0, -1, 1, 0, -1};
//const int dx[8] = {2, 2, 1, 1, -1, -1, -2, -2};
//const int dy[8] = {1, -1, 2, -2, 2, -2, 1, -1};

const int mod = (int)1e9 + 7;
//const int MAX_MEM = 1e8;
//int mpos = 0;
//char mem[MAX_MEM];
//inline void * operator new ( size_t n ) {
//    assert((mpos += n) <= MAX_MEM);
//    return (void *)(mem + mpos - n);
//}
//inline void operator delete ( void * ) noexcept { }
char a[21][21];
signed main() {
    ios_base::sync_with_stdio(false); cin.tie(0); cout.tie(0);
    #ifdef KULA
            freopen("input.txt", "r", stdin);
//            freopen("output.txt", "w", stdout);
    #else
//            freopen("mushrooms.in", "r", stdin);
//            freopen("mushrooms.out", "w", stdout);
    #endif
    int n, m;
    cin >> n >> m;
    forn(i, n){
        forn(j, m){
            cin >> a[i][j];
        }
    }
    int ans = -1;
    forn(i, n){
        forn(j, m){
            if (a[i][j] == '#') cont;
            queue<pii> q;
            q.push({i, j});
            vvi d(n, vi(m, INT_MAX));
            d[i][j] = 0;
            int gg = 0;
            while(q.size()){
                pii now = q.front();
                q.pop();
                forn(i, 4){
                    int nx = now.X + dx[i];
                    int ny = now.Y + dy[i];
                    if (nx >=0 && nx < n && ny >= 0 && ny < m && d[nx][ny] == INT_MAX && a[nx][ny] == '.'){
                        d[nx][ny] = d[now.X][now.Y] + 1;
                        uax(gg, d[nx][ny]);
                        q.push({nx, ny});
                    }
                }
            }
            uax(ans, gg);
        }
    }
    cout << ans;

    return 0;
}


Measured memory_bytes: 256