#pragma GCC optimize ("O3")
#include "bits/stdc++.h"

using namespace std;
using ll = long long int;

#define debugos cout
#define debug(v) {printf("L%d %s > ",__LINE__,#v);debugos<<(v)<<endl;}
#define debugv(v) {printf("L%d %s > ",__LINE__,#v);for(auto e:(v)){debugos<<e<<" ";}debugos<<endl;}
#define debuga(m,w) {printf("L%d %s > ",__LINE__,#m);for(int x=0;x<(w);x++){debugos<<(m)[x]<<" ";}debugos<<endl;}
#define debugaa(m,h,w) {printf("L%d %s >\n",__LINE__,#m);for(int y=0;y<(h);y++){for(int x=0;x<(w);x++){debugos<<(m)[y][x]<<" ";}debugos<<endl;}}
#define all(v) (v).begin(),(v).end()
#define repeat(cnt,l) for(auto cnt=remove_reference<decltype(l)>::type();(cnt)<(l);++(cnt))
#define rrepeat(cnt,l) for(auto cnt=(l)-1;0<=(cnt);--(cnt))
#define iterate(cnt,b,e) for(auto cnt=(b);(cnt)!=(e);++(cnt))
#define diterate(cnt,b,e) for(auto cnt=(b);(cnt)!=(e);--(cnt))
const ll MD = 1000000007ll; const long double PI = 3.1415926535897932384626433832795L;
inline void assert_call(bool assertion, function<void()> f) { if (!assertion) { cerr << "assertion fault:" << endl; f(); abort(); } }
template<typename T1, typename T2> inline ostream& operator <<(ostream &o, const pair<T1, T2> p) { o << '(' << p.first << ':' << p.second << ')'; return o; }
template<typename Vec> inline ostream& _ostream_vecprint(ostream& os, const Vec& a) {
  os << '['; for (const auto& e : a) os << ' ' << e << ' '; os << ']'; return os; }
template<typename T> inline ostream& operator<<(ostream& o, const vector<T>& v) { return _ostream_vecprint(o, v); }
template<typename T, size_t S> inline ostream& operator<<(ostream& o, const array<T, S>& v) { return _ostream_vecprint(o, v); }
template<typename T> inline T& chmax(T& to, const T& val) { return to = max(to, val); }
template<typename T> inline T& chmin(T& to, const T& val) { return to = min(to, val); }
void bye(string s, int code = 0) { cout << s << endl; exit(code); }
mt19937_64 randdev(8901016);
template<typename T, typename Random = decltype(randdev), typename enable_if<is_integral<T>::value>::type* = nullptr>
inline T rand(T l, T h, Random& rand = randdev) { return uniform_int_distribution<T>(l, h)(rand); }
template<typename T, typename Random = decltype(randdev), typename enable_if<is_floating_point<T>::value>::type* = nullptr>
inline T rand(T l, T h, Random& rand = randdev) { return uniform_real_distribution<T>(l, h)(rand); }

#if defined(_WIN32) || defined(_WIN64)
#define getc_x _getc_nolock
#define putc_x _putc_nolock
#elif defined(__GNUC__)
#define getc_x getc_unlocked
#define putc_x putc_unlocked
#else
#define getc_x getc
#define putc_x putc
#endif
#define isvisiblechar(c) (0x21<=(c)&&(c)<=0x7E)
class MaiScanner {
  FILE* fp_;
public:
  inline MaiScanner(FILE* fp):fp_(fp){}
  template<typename T> void input_integer(T& var) noexcept {
    var = 0; T sign = 1;
    int cc = getc_x(fp_);
    for (; cc < '0' || '9' < cc; cc = getc_x(fp_))
      if (cc == '-') sign = -1;
    for (; '0' <= cc && cc <= '9'; cc = getc_x(fp_))
      var = (var << 3) + (var << 1) + cc - '0';
    var = var * sign;
  }
  inline int c() noexcept { return getc_x(fp_); }
  inline MaiScanner& operator>>(int& var) noexcept { input_integer<int>(var); return *this; }
  inline MaiScanner& operator>>(long long& var) noexcept { input_integer<long long>(var); return *this; }
  inline MaiScanner& operator>>(string& var) {
    int cc = getc_x(fp_);
    for (; !isvisiblechar(cc); cc = getc_x(fp_));
    for (; isvisiblechar(cc); cc = getc_x(fp_))
      var.push_back(cc);
    return *this;
  }
  template<typename IT> inline void in(IT begin, IT end) { for (auto it = begin; it != end; ++it) *this >> *it; }
};
class MaiPrinter {
  FILE* fp_;
public:
  inline MaiPrinter(FILE* fp):fp_(fp){}
  template<typename T>
  void output_integer(T var) noexcept {
    if (var == 0) { putc_x('0', fp_); return; }
    if (var < 0)
      putc_x('-', fp_),
      var = -var;
    char stack[32]; int stack_p = 0;
    while (var)
      stack[stack_p++] = '0' + (var % 10),
      var /= 10;
    while (stack_p)
      putc_x(stack[--stack_p], fp_);
  }
  inline MaiPrinter& operator<<(char c) noexcept { putc_x(c, fp_); return *this; }
  inline MaiPrinter& operator<<(int var) noexcept { output_integer<int>(var); return *this; }
  inline MaiPrinter& operator<<(long long var) noexcept { output_integer<long long>(var); return *this; }
  inline MaiPrinter& operator<<(char* str_p) noexcept { while (*str_p) putc_x(*(str_p++), fp_); return *this; }
  inline MaiPrinter& operator<<(const string& str) {
    const char* p = str.c_str();
    const char* l = p + str.size();
    while (p < l) putc_x(*p++, fp_);
    return *this;
  }
  template<typename IT> void join(IT begin, IT end, char sep = ' ') { for (bool b = 0; begin != end; ++begin, b = 1) b ? *this << sep << *begin : *this << *begin; }
};
MaiScanner scanner(stdin);
MaiPrinter printer(stdout);

class Graph2d {
 public:
  using W_T = int;
  int n;
  vector<W_T> matrix;

  explicit Graph2d(int size) : n(size), matrix(size * size){};

  inline int size() const { return n; }
  void resize(int s) {
    n = s;
    matrix.resize(n * n);
  }
  void resize(int s, W_T val) {
    n = s;
    matrix.resize(n * n, val);
  }

  inline W_T& at(int y, int x) { return matrix[y * n + x]; }
  inline W_T& operator()(int y, int x) { return matrix[y * n + x]; }
  inline W_T at(int y, int x) const { return matrix[y * n + x]; }
  inline W_T operator()(int y, int x) const { return matrix[y * n + x]; }

  inline void connect(int u, int v, W_T dist = 1) { at(u, v) = at(v, u) = dist; }
  inline void connect_d(int from, int to, W_T dist = 1) {  // directedEdge u->v
    at(from, to) = dist;
  }
};

void warshall_floyd(Graph2d& g) {
  int i, j, k;
  for (i = 0; i < g.n; i++) {
    for (j = 0; j < g.n; j++) {
      for (k = 0; k < g.n; k++) {
        g(j, k) = std::min(g(j, k), g(j, i) + g(i, k));
      }
    }
  }
}


//

int H, W;
string field[22];

//

constexpr int inf = 1e9;


int main() {
  
  scanner >> H >> W;
  scanner.in(field, field + H);
  repeat(i, H) {
    field[i].push_back('#');
  }
  repeat(i, W) field[H].push_back('#');
  
  Graph2d mat(H*W);
  
  fill(all(mat.matrix), inf);
  repeat(i, H*W) mat(i, i) = 0;
  
  repeat(y, H) {
    repeat(x, W) {
      if (field[y][x] == '#') continue;
      if (field[y][x+1] == '.') {
        mat(W*y+x, W*y+x+1) = 1;
        mat(W*y+x+1, W*y+x) = 1;
      }
      if (field[y+1][x] == '.') {
        mat(W*y+x, W*y+x+W) = 1;
        mat(W*y+x+W, W*y+x) = 1;
      }
    }
  }
  
  warshall_floyd(mat);
  
  int far = 0;
  for (int d : mat.matrix) {
    if (d != inf) chmax(far, d);
  }
  
  cout << far << endl;
  
  return 0;
}


Measured memory_bytes: 896