#include <bits/stdc++.h>
using namespace std;

// #include <boost/algorithm/string/join.hpp>
// using boost::algorithm::join;
//
// #include "boost/multi_array.hpp"
// boost::multi_array<int, 3> dp;
// dp.resize(boost::extents[10][9][8]);
//
// #include <boost/multiprecision/cpp_dec_float.hpp>  // quite slow
// #include <boost/multiprecision/cpp_int.hpp>
// namespace mp = boost::multiprecision; // mp::log10
// using bigint = boost::multiprecision::cpp_int;

typedef long long ll;
typedef unsigned long long ull;
//typedef long double ld;

typedef map<int, int> mii;
typedef pair<int, int> pii;
typedef pair<double,double> pdd;
typedef set<int> si;
typedef set<ll> sll;
typedef vector<int> vi;
typedef vector<pair<int, int > > vii;
typedef vector<ll> vll;
typedef vector<double> vd;
typedef vector<vi> vvi;
typedef vector<vii> vvii;
typedef vector<vll> vvll;

//constexpr ll MOD = 1e9+7;
template<typename T>constexpr auto inf=numeric_limits<T>::max()/4;
constexpr int INF=inf<int>;
constexpr ll LINF=inf<ll>;

#define Sort(x) sort(begin(x),end(x))
#define Reverse(x) reverse(begin(x),end(x))
#define ABS(a,b) ((a)<(b)?(b)-(a):(a)-(b))
#define nbit(n) (1ull<<(n))
#define _ol3(_1,_2,_3,name,...)name
#define _rep(i,n)_repi(i,0,n)
#define _repi(i,a,b)for(int i=a,i##_l=(b);i<i##_l;++i)
#define REP(...) _ol3(__VA_ARGS__,_repi,_rep,)(__VA_ARGS__)
#define REPR(i,n) for(int i=n-1;i>=0;--i)
#define REPA(i,v) REP(i,(v).size())

// sort(begin(x), end(x), greater<ll>());
// lower_bound(a.begin(), a.end(), val): a[i] ≥ valである最左の位置
// upper_bound(a.begin(), a.end(), val): a[i] > valである最左の位置

// Returns minimum of a and b.
// If a is less b, a is set to b.
template<typename T>
T chmin(T& a, T b) {
  if (a > b) {
    a = b;
  }
  return a;
}

// Returns maximum of a and b.
// If a is less b, a is set to b.
template<typename T>
T chmax(T& a, T b) {
  if (a < b) {
    a = b;
  }
  return a;
}

template <typename T>
void dumpContents(const T& v, const string& msg)
{
  cerr << "### " << msg << " ###\n";
  for (const auto& x : v) {
    cerr << x << " ";
  }
  cerr << endl;
}

struct before_main_function {
  before_main_function() {
    cin.tie(0);
    ios::sync_with_stdio(false);
    cout<<setprecision(20)<<fixed;
#define endl "\n"
  }
} before_main_function;

// ========== end of template ==========
int H, W;
int N;

int to_id(int h, int w) {
  if (h<0 || h>=H || w<0 || w>=W) return -1;

  return h*W + w;
}

pii to_hw(int id) {
  assert(0<=id && id<N);

  int h = id/W;
  int w = id%W;
  return {h,w};
}


int main(int argc, char** argv) {
  cin >> H >> W;

  vector<string> M(H);
  for (auto& s : M) {
    cin >> s;
  }

  N=H*W;
  vvi d(N, vi(N, INF));
  REP(i, N) {
    d[i][i] = 0;
  }

  REP(h, H) {
    REP(w, W) {
      const char c = M[h][w];
      if (c == '#') continue;

      const int id = to_id(h, w);

      int up = to_id(h-1, w);
      if (up>=0) {
        pii hw = to_hw(up);
        char c = M[hw.first][hw.second];
        if (c == '.') {
          d[id][up] = 1;
          d[up][id] = 1;
        }
      }

      int down = to_id(h+1, w);
      if (down>=0) {
        pii hw = to_hw(down);
        char c = M[hw.first][hw.second];
        if (c == '.') {
          d[id][down] = 1;
          d[down][id] = 1;
        }
      }

      int left = to_id(h, w-1);
      if (left>=0) {
        pii hw = to_hw(left);
        char c = M[hw.first][hw.second];
        if (c == '.') {
          d[id][left] = 1;
          d[left][id] = 1;
        }
      }

      int right = to_id(h, w+1);
      if (right>=0) {
        pii hw = to_hw(right);
        char c = M[hw.first][hw.second];
        if (c == '.') {
          d[id][right] = 1;
          d[right][id] = 1;
        }
      }

    }
  }

  REP(k, N) {
    REP(i, N) {
      REP(j, N) {
        chmin(d[i][j], d[i][k]+d[k][j]);
      }
    }
  }

  int ret = 0;
  REP(i, N) {
    REP(j, N) {
      if (d[i][j] < INF) {
        chmax(ret, d[i][j]);
      }
    }
  }

  cout << ret << endl;
  return 0;
}

/* vim: set expandtab shiftwidth=2 softtabstop=2 : */


Measured memory_bytes: 896