#ifndef DEBUG
#  define NDEBUG
#endif
#include <iostream>
#include <string>
#include <iterator>
#include <utility>
#include <type_traits>
#include <limits>
#include <cassert>
#include <vector>
#include <queue>
#include <tuple>

using namespace std;

// template {{{
// misc {{{
using Z = long long;
[[maybe_unused]]
constexpr Z INF = numeric_limits<Z>::max() / 3;
// }}}

// debug {{{
template <class T>
using enable_if_sequence_t = enable_if_t<
  is_same<
    typename iterator_traits<
      typename remove_cv_t<remove_reference_t<T>>::iterator
    >::iterator_category,
    random_access_iterator_tag>{}>;
#ifdef NDEBUG
# define dump(x)
# define debug if (0)
template <class T>
inline T&& ix(T&& value) { return value; }
template <class T, class Index, class... Indices, class = enable_if_sequence_t<T>>
inline auto&& ix(T&& container, Index index, Indices... indices) {
  return ix(container[index], indices...);
}
#else
# define dump(x) do { \
    cerr << "\e[33m[dump: " << __LINE__ << "] " << (#x) << " = " << x << "\e[0m" << endl; \
  } while (0);
# define debug
template <class T>
inline auto&& ix_impl([[maybe_unused]] int line, [[maybe_unused]] const char* code, T&& value) {
  return value;
}
template <class T, class Index, class... Indices, class = enable_if_sequence_t<T>>
inline auto&& ix_impl(int line, const char* code, T&& container, Index index, Indices... indices) {
  if (index < 0) {
    cerr << "\e[31mix: negative index at line " << line << ", ix(" << code << ")\e[0m" << endl;
    exit(1);
  }
  int size = container.size();
  if (index >= size) {
    cerr << "\e[31mix: index out of range at line " << line << ", ix(" << code << ")\n"
      << "index = " << index << " whereas size = " << size << "\e[0m" << endl;
    exit(1);
  }
  return ix_impl(line, code, container[index], indices...);
}
#define ix(...) ix_impl(__LINE__, #__VA_ARGS__, __VA_ARGS__)
#endif
// }}}

// IO {{{
template <class T, class U>
ostream& operator<< (ostream& os, pair<T, U> p) {
  os << '(' << p.first << ", " << p.second << ')';
  return os;
}
template <class T>
false_type is_container_impl(...);
template <class T, class = typename T::iterator>
true_type is_container_impl(T);
template <class T>
struct is_container : decltype(is_container_impl<T>(declval<T>())) {};
template <>
struct is_container <string> : std::false_type {};
template <class Container, class = enable_if_t<is_container<Container>{}>>
istream& operator>> (istream& is, Container& cont) {
  for (auto& v : cont) is >> v;
  return is;
}
template <class Container, class = enable_if_t<is_container<Container>{}>>
ostream& operator<< (ostream& os, Container const& cont) {
  bool first = true;
  constexpr char sep =
    is_container<typename Container::value_type>{} ? '\n' : ' ';
  for (auto const& v : cont) {
    if (!first) {
      os << sep;
    }
    first = false;
    os << v;
  }
  return os;
}
// }}}
// }}}

int main() {
  ios_base::sync_with_stdio(false);
  cin.tie(nullptr);
  Z H, W;
  cin >> H >> W;
  vector<string> field(H + 2);
  ix(field, 0) = string(W + 2, '#');
  ix(field, H + 2 - 1) = string(W + 2, '#');
  for (auto i = 1; i <= H; i++) {
    string s;
    cin >> s;
    ix(field, i) = '#' + s + '#';
  }
  Z ans = 0;
  for (auto i = 1; i <= H; i++) {
    for (auto j = 1; j <= W; j++) {
      if (ix(field, i, j) == '#') continue;
      vector<vector<Z>> dists(H + 2, vector<Z>(W + 2, INF));
      queue<pair<Z, Z>> que;
      ix(dists, i, j) = 0;
      que.emplace(i, j);
      while (!que.empty()) {
        Z x, y;
        tie(x, y) = que.front();
        que.pop();
        vector<Z> dx = {1, 0, -1, 0}, dy = {0, 1, 0, -1};
        for (auto i = 0; i < 4; i++) {
          Z nx = x + ix(dx, i), ny = y + ix(dy, i);
          if (ix(field, nx, ny) == '#' || ix(dists, nx, ny) < INF)
            continue;
          ix(dists, nx, ny) = ix(dists, x, y) + 1;
          que.emplace(nx, ny);
        }
      }
      for (auto&& row : dists)
        for (auto&& d : row)
          if (d < INF) ans = max(ans, d);
    }
  }
  cout << ans << endl;
}



Measured memory_bytes: 256