#include <bits/stdc++.h>
using namespace std;
#define REP(i,b,e) for(int i=(b);i<(e);++i)
#define RREP(i,b,e) for(int i=(b)-1;i>=e;--i)
#define rep(i,e) for(int i=0;i<(e);++i)

constexpr int MOD = 1000000007;
constexpr int INF = 1LL << 30;
constexpr long long LLINF = 1LL << 62;
constexpr double EPS = 1e-9;

struct Initializer {
  Initializer() {
    cin.tie(0);
    ios::sync_with_stdio(0);
    cout << fixed << setprecision(15) << boolalpha;
  }
} initializer;

template<typename T>
istream &operator>>(istream &is, vector<T> &vec) {
  for (T &x : vec) is >> x;
  return is;
}

template<typename T>
ostream &operator<<(ostream &os, const vector<T> &vec) {
  os << "[";
  for (auto it = vec.begin(); it != vec.end(); ++it) {
    if (it != vec.begin()) os << ", ";
    os << *it;
  }
  return os << "]";
}

template<typename T, typename U>
ostream &operator<<(ostream &os, const pair<T, U> &pa) {
  return os << "(" << pa.first << ", " << pa.second << ")";
}

template<typename Tuple, size_t N>
struct TuplePrinter {
  static void print(const Tuple &t) {
    TuplePrinter<Tuple, N-1>::print(t);
    cout << ", " << get<N-1>(t);
  }
};

template<typename Tuple>
struct TuplePrinter<Tuple, 1> {
  static void print(const Tuple &t) {
    cout << get<0>(t);
  }
};

template<typename... T>
ostream &operator<<(ostream &os, const tuple<T...> &tup) {
  os << "(";
  TuplePrinter<decltype(tup), sizeof...(T)>::print(tup);
  return os << ")";
}

template<typename T>
ostream &operator<<(ostream &os, const set<T> &se) {
  os << "{";
  for (auto it = se.begin(); it != se.end(); ++it) {
    if (it != se.begin()) os << ", ";
    os << *it;
  }
  return os << "}";
}

template<typename T, typename U>
ostream &operator<<(ostream &os, const map<T, U> &ma) {
  os << "{";
  for (auto it = ma.begin(); it != ma.end(); ++it) {
    if (it != ma.begin()) os << ", ";
    os << it->first << ": " << it->second;
  }
  return os << "}";
}

inline void print(void) { cout<<'\n'; }
template<class T> inline void print(const T &x) { cout<<x<<'\n'; }
template<class T, class... U> inline void print(const T &x, const U&... y) { cout<<x<<" "; print(y...); }
#define dump(...) cout<<#__VA_ARGS__<<": [L_"<<__LINE__<<"]"<<'\n'; print(__VA_ARGS__);cout<<'\n';

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

void bfs(int root, vector<vector<int>> &g, vector<int> &dists) {
  queue<int> que;
  que.push(root);
  dists[root] = 0;
  while (!que.empty()) {
    int x = que.front(); que.pop();
    for (auto nx: g[x]) {
      if (dists[nx] > dists[x] + 1) {
        dists[nx] = dists[x] + 1;
        que.push(nx);
      }
    }
  }
}

int main() {
  int h, w; cin>>h>>w;
  vector<string> s(h);
  rep(i, h) cin>>s[i];

  vector<vector<int>> g(h*w);
  rep(i, h) rep(j, w) {
    if (j != w-1 and s[i][j+1] != '#') g[i*w+j].emplace_back(i*w+j+1);
    if (j != 0 and s[i][j-1] != '#') g[i*w+j].emplace_back(i*w+j-1);
    if (i != h-1 and s[i+1][j] != '#') g[i*w+j].emplace_back((i+1)*w+j);
    if (i != 0 and s[i-1][j] != '#') g[i*w+j].emplace_back((i-1)*w+j);
  }

  int ans = 0;

  rep(i, h) rep(j, w) if (s[i][j] != '#') {
    vector<int> dists(h*w, INF);
    bfs(i*w+j, g, dists);
    rep(i, h) rep(j, w) if (dists[i*w+j] != INF) ans = max(ans, dists[i*w+j]);
  }
  print(ans);

  return 0;
}

Measured memory_bytes: 256