#ifdef LOCAL
    #define _GLIBCXX_DEBUG
    #define __clock__
#else
    #pragma GCC optimize("Ofast")
#endif
#include<bits/stdc++.h>
using namespace std;
using ll = long long;
using VI = vector<ll>;
using VV = vector<VI>;
using VS = vector<string>;
using PII = pair<ll, ll>;

// tourist set
template <typename A, typename B>
string to_string(pair<A, B> p);

template <typename A, typename B, typename C>
string to_string(tuple<A, B, C> p);

template <typename A, typename B, typename C, typename D>
string to_string(tuple<A, B, C, D> p);

string to_string(const string& s) {
  return '"' + s + '"';
}

string to_string(const char* s) {
  return to_string((string) s);
}

string to_string(bool b) {
  return (b ? "true" : "false");
}

string to_string(vector<bool> v) {
  bool first = true;
  string res = "{";
  for (int i = 0; i < static_cast<int>(v.size()); i++) {
    if (!first) {
      res += ", ";
    }
    first = false;
    res += to_string(v[i]);
  }
  res += "}";
  return res;
}

template <size_t N>
string to_string(bitset<N> v) {
  string res = "";
  for (size_t i = 0; i < N; i++) {
    res += static_cast<char>('0' + v[i]);
  }
  return res;
}

template <typename A>
string to_string(A v) {
  bool first = true;
  string res = "{";
  for (const auto &x : v) {
    if (!first) {
      res += ", ";
    }
    first = false;
    res += to_string(x);
  }
  res += "}";
  return res;
}

template <typename A, typename B>
string to_string(pair<A, B> p) {
  return "(" + to_string(p.first) + ", " + to_string(p.second) + ")";
}

template <typename A, typename B, typename C>
string to_string(tuple<A, B, C> p) {
  return "(" + to_string(get<0>(p)) + ", " + to_string(get<1>(p)) + ", " + to_string(get<2>(p)) + ")";
}

template <typename A, typename B, typename C, typename D>
string to_string(tuple<A, B, C, D> p) {
  return "(" + to_string(get<0>(p)) + ", " + to_string(get<1>(p)) + ", " + to_string(get<2>(p)) + ", " + to_string(get<3>(p)) + ")";
}

void debug_out() { cerr << '\n'; }

template <typename Head, typename... Tail>
void debug_out(Head H, Tail... T) {
  cerr << " " << to_string(H);
  debug_out(T...);
}

#ifdef LOCAL
#define debug(...) cerr << "[" << #__VA_ARGS__ << "]:", debug_out(__VA_ARGS__)
#else
#define debug(...) 42
#endif
// tourist set end

template<class T>bool chmax(T &a, const T &b) { if (a<b) { a=b; return 1; } return 0; }
template<class T>bool chmin(T &a, const T &b) { if (b<a) { a=b; return 1; } return 0; }

#define FOR(i,a,b) for(ll i=(a);i<(b);++i)
#define rep(i,b) FOR(i, 0, b)
#define ALL(v) (v).begin(), (v).end()
#define p(s) cout<<(s)<<'\n'
#define p2(s, t) cout << (s) << " " << (t) << '\n'
#define br() p("")
#define pn(s) cout << (#s) << " " << (s) << '\n'
#define p_yes() p("YES")
#define p_no() p("NO")
#define SZ(x) ((int)(x).size())
#define SORT(A) sort(ALL(A))
#define RSORT(A) sort(ALL(A), greater<ll>())
#define MP make_pair

void no(){p_no(); exit(0);}
void yes(){p_yes(); exit(0);}

const ll mod = 1e9 + 7;
const ll inf = 1e18;
const double PI = acos(-1);

ll d[30][30];
void reset_d(){
  rep(i, 30){
    rep(j, 30){
      d[i][j] = inf;
    }
  }
}

int dx[4] = {-1, 0, 1,  0};
int dy[4] = { 0, 1, 0, -1};

ll H,W;

bool in_range(ll i, ll j){
  if(0<=i && i<H && 0<=j && j<W){
    return true;
  }else{
    return false;
  }
}

int main(){
    cin.tie(0);
    ios::sync_with_stdio(false);

    // input
    cin >> H>>W;

    vector<string> M(H);
    rep(i, H){
      cin >> M[i];
    }

    vector<PII> candidates;
    // どこか点
    rep(i, H){
      rep(j, W){
        if(M[i][j]=='.'){
          candidates.push_back(MP(i, j));
        }
      }
    }

    ll ma = 0;
    for(auto start : candidates){
      reset_d();
      d[start.first][start.second] = 0;
      queue<PII> que;
      que.push(start);
      while(!que.empty()){
        auto pa = que.front(); que.pop();
        ll y = pa.first;
        ll x = pa.second;
        ll dist = d[y][x];
        
        rep(i, 4){
          ll ty = y + dy[i];
          ll tx = x + dx[i];
          if(in_range(ty, tx) && M[ty][tx]=='.' && d[ty][tx]==inf){
            que.push(MP(ty, tx));
            d[ty][tx] = dist + 1;
          }
        }
      }

      rep(i, H){
        rep(j, W){
          if(d[i][j]==inf) continue;
          chmax(ma, d[i][j]);
        }
      }
    }
    p(ma);
    
    return 0;
}

Measured memory_bytes: 256