#define DEBUG 0
#include <bits/stdc++.h>
#define loop(n) for (lint ngtkana_is_a_genius = 0; ngtkana_is_a_genius < lint(n); ngtkana_is_a_genius++)
#define rep(i, begin, end) for (lint i = lint(begin); (i) < lint(end); i++)
#define all(v) v.begin(), v.end()
#define rand(l, r) std::uniform_int_distribution<>(l, r)(mt)
using lint = long long;
auto mt = std::mt19937_64(std::random_device{}());
auto cmn = [](auto&& a, auto b){ if (a > b) {a = b; return true;} return false; };
auto cmx = [](auto&& a, auto b){ if (a < b) {a = b; return true;} return false; };
void debug_impl() { std::cerr << std::endl; }
template <typename Head, typename... Tail>
void debug_impl(Head head, Tail... tail) { std::cerr << " " << head; debug_impl(tail...); }
#if DEBUG
#define debug(...)\
  do {\
    std::cerr << std::boolalpha << "[" << #__VA_ARGS__ << "]:";\
    debug_impl(__VA_ARGS__);\
    std::cerr << std::noboolalpha;\
  } while (false)
#else
#define debug(...) {}
#endif

template < typename Container, typename Value = typename Container::value_type, std::enable_if_t<!std::is_same< Container, std::string >::value, std::nullptr_t> = nullptr>
std::istream& operator>> (std::istream& is, Container& v)
  { for (auto & x : v) { is >> x; } return is; }

template < typename Container, typename Value = typename Container::value_type, std::enable_if_t<!std::is_same< Container, std::string >::value, std::nullptr_t> = nullptr >
std::ostream& operator<< (std::ostream& os, Container const& v) {
 os << "{";
  for (auto it = v.begin(); it != v.end(); it++)
    {os << (it != v.begin() ? "," : "") << *it;}
  return os << "}";
}

template <typename T>
auto make_vector_impl(size_t sz, T t) {return std::vector<T>(sz, t);}

template <size_t N, typename T, typename U, std::enable_if_t<N == 1, std::nullptr_t> = nullptr>
auto make_vector(size_t sz, U u) {return make_vector_impl(sz, T(u));}

template <size_t N, typename T, std::enable_if_t<N == 1, std::nullptr_t> = nullptr>
auto make_vector(size_t sz) {return std::vector<T>(sz);}

template <size_t N, typename T, typename... Args, std::enable_if_t<N != 1, std::nullptr_t> = nullptr>
auto make_vector(size_t a, Args... args) {return make_vector_impl(a, make_vector<N - 1, T>(args...));}

template <typename T, typename Size_t>
auto& at(T& t, Size_t i) {return t.at(i);}

template <typename T, typename Size_t, typename... Args>
auto& at(T& t, Size_t i, Args... args) {return at(t.at(i), args...);}

int di[4] = {-1, +1, 0, 0};
int dj[4] = {0, 0, -1, +1};

template <class Key>
class fmt_t {
    Key key;

    template <class Value>
    auto is_exact_match(long, Value) const {
      return false;
    }

    template <class Value, class = decltype(std::declval< Value >() == key)>
    auto is_exact_match(int, Value x) const {
      return x == key;
    }


    // tuple_format_impl
    template < class Tuple,  std::size_t ... Is >
    std::string tuple_format_impl(const Tuple& tuple, std::integer_sequence< std::size_t, Is ... >) const {
      std::string ret = "(";
      (void)std::initializer_list< std::nullptr_t >
        { (ret = ret + (Is == 0 ? "" : ",") + format(std::get< Is >(tuple)), nullptr) ... };
      return ret + ")";
    }

    // usual_format
    template < class Value >
    std::string usual_format(long, Value x) const
      { return std::to_string(x); }

    template < class Value, std::enable_if_t< std::is_same< std::decay_t< Value >, char >::value, std::nullptr_t > = nullptr >
    std::string usual_format(int, Value c) const
      { return std::string{'\''} + std::string{c} + std::string{'\''}; }

    std::string usual_format(int, char const* s) const
      { return "\"" + std::string(s) + "\""; }

    std::string usual_format(int, std::string const s) const
      { return "\"" + s + "\""; }

    template < class Container, class = typename Container::value_type >
    std::string usual_format(int, Container const & v) const {
      return "{" + std::accumulate(v.begin(), v .end(), std::string{},
        [this](auto const& s, auto const x) { return s + (s.empty() ? "" : ",") + this->format(x); })
        + "}";
    }

    template < class Tuple, std::size_t N = std::tuple_size< Tuple >::value >
    std::string usual_format(int, Tuple const& tuple) const
      { return tuple_format_impl(tuple, std::make_index_sequence< N >()); }

    // format
    template < class Value > std::string format(Value&& x) const
      { return is_exact_match(int{}, x) ? std::string{'_'} : usual_format(int{}, x); }

  public:
    fmt_t()=default;
    fmt_t(Key&& key) : key(key){}

    template < class Value > std::string operator()(Value&& x) const
      { return format(x); }
};
template <class Key> auto fmt(Key key)
  { return fmt_t<Key>(std::forward<Key>(key)); }

int main() {
  std::cin.tie(0); std::cin.sync_with_stdio(false);
  int h, w; std::cin >> h >> w;
  auto grid = make_vector<2,int>(h,w);
  rep(i,0,h) rep(j,0,w) {
    char c; std::cin >> c;
    at(grid,i,j) = c=='#';
  }

  auto cal = [&] (int si, int sj) {
    auto dist = make_vector<2,int>(h,w,-1);
    std::queue<std::pair<int,int>> que;
    que.emplace(si,sj);
    at(dist,si,sj) = 0;
    while (!que.empty()) {
      int i,j; std::tie(i,j) = que.front(); que.pop();
      rep(k,0,4) {
        int ni = i + di[k];
        int nj = j + dj[k];
        if (ni < 0 || h <= ni || nj < 0 || w <= nj || at(grid,ni,nj) || at(dist,ni,nj) != -1) {
          continue;
        }
        at(dist,ni,nj) = at(dist,i,j) + 1;
        que.emplace(ni,nj);
      }
    }

    #if DEBUG
      rep(i,0,h) {
        auto v = fmt(-1)(dist.at(i));
        debug(i,v);
      }
      std::cerr<<std::endl;
    #endif

    int ans = 0;
    for (auto&& v : dist) for (int x : v) {
      cmx(ans, x);
    }
    return ans;
  };

  int ans = 0;
  rep(i,0,h) rep(j,0,w) {
    if (at(grid,i,j)) continue;
    int now = cal(i,j);
    debug(i,j,now);
    cmx(ans, now);
  }
  std::cout << ans << std::endl;
  return 0;
}

Measured memory_bytes: 256