#include <bits/stdc++.h>

using namespace std;

using int64 = long long;
const int mod = 1e9 + 7;
// const int mod = 998244353;

const int64 infll = (1LL << 62) - 1;
const int inf = (1 << 30) - 1;

struct IoSetup {
  IoSetup() {
    cin.tie(nullptr);
    ios::sync_with_stdio(false);
    cout << fixed << setprecision(10);
    cerr << fixed << setprecision(10);
  }
} iosetup;


template< typename T1, typename T2 >
ostream &operator<<(ostream &os, const pair< T1, T2 > &p) {
  os << p.first << " " << p.second;
  return os;
}

template< typename T1, typename T2 >
istream &operator>>(istream &is, pair< T1, T2 > &p) {
  is >> p.first >> p.second;
  return is;
}

template< typename T >
ostream &operator<<(ostream &os, const vector< T > &v) {
  for(int i = 0; i < (int) v.size(); i++) {
    os << v[i] << (i + 1 != v.size() ? " " : "");
  }
  return os;
}

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

template< typename T1, typename T2 >
inline bool chmax(T1 &a, T2 b) { return a < b && (a = b, true); }

template< typename T1, typename T2 >
inline bool chmin(T1 &a, T2 b) { return a > b && (a = b, true); }

template< typename T = int64 >
vector< T > make_v(size_t a) {
  return vector< T >(a);
}

template< typename T, typename... Ts >
auto make_v(size_t a, Ts... ts) {
  return vector< decltype(make_v< T >(ts...)) >(a, make_v< T >(ts...));
}

template< typename T, typename V >
typename enable_if< is_class< T >::value == 0 >::type fill_v(T &t, const V &v) {
  t = v;
}

template< typename T, typename V >
typename enable_if< is_class< T >::value != 0 >::type fill_v(T &t, const V &v) {
  for(auto &e : t) fill_v(e, v);
}

template< typename F >
struct FixPoint : F {
  FixPoint(F &&f) : F(forward< F >(f)) {}

  template< typename... Args >
  decltype(auto) operator()(Args &&... args) const {
    return F::operator()(*this, forward< Args >(args)...);
  }
};

template< typename F >
inline decltype(auto) MFP(F &&f) {
  return FixPoint< F >{forward< F >(f)};
}

vector< vector< int > > grid_bfs(vector< string > &s, char start, const string &wall = "#") {
  const int vx[] = {0, 1, 0, -1}, vy[] = {1, 0, -1, 0};
  vector< vector< int > > min_cost(s.size(), vector< int >(s[0].size(), -1));
  queue< pair< int, int > > que;
  for(int i = 0; i < s.size(); i++) {
    for(int j = 0; j < s[i].size(); j++) {
      if(s[i][j] == start) {
        que.emplace(i, j);
        min_cost[i][j] = 0;
      }
    }
  }
  while(!que.empty()) {
    auto p = que.front();
    que.pop();
    for(int i = 0; i < 4; i++) {
      int ny = p.first + vy[i], nx = p.second + vx[i];
      if(nx < 0 || ny < 0 || nx >= s[0].size() || ny >= s.size()) continue;
      if(min_cost[ny][nx] != -1) continue;
      if(wall.find(s[ny][nx]) != string::npos) continue;
      min_cost[ny][nx] = min_cost[p.first][p.second] + 1;
      que.emplace(ny, nx);
    }
  }
  return min_cost;
}

int main() {
  int H, W;
  cin >> H >> W;
  vector< string > S(H);
  cin >> S;
  int ret = 0;
  for(int i = 0; i < H; i++) {
    for(int j = 0; j < W; j++) {
      if(S[i][j] == '.') {
        S[i][j] = 'S';
        auto D = grid_bfs(S, 'S', "#");
        for(auto &k : D) chmax(ret, *max_element(begin(k), end(k)));
        S[i][j] = '.';
      }
    }
  }
  cout << ret << endl;
}


Measured memory_bytes: 256