#include <bits/stdc++.h>
using namespace std;
using i32 = int;
using i64 = long long;
using u64 = unsigned long long;
using f80 = long double;
using vi32 = vector<i32>;
using vi64 = vector<i64>;
using vu64 = vector<u64>;
using vf80 = vector<f80>;
using vstr = vector<string>;
inline void yes() {
  cout << "Yes" << '\n';
  exit(0);
}
inline void no() {
  cout << "No" << '\n';
  exit(0);
}
inline i64 gcd(i64 a, i64 b) {
  if (min(a, b) == 0) return max(a, b);
  if (a % b == 0) return b;
  return gcd(b, a % b);
}
inline i64 lcm(i64 a, i64 b) { return a / gcd(a, b) * b; }
inline u64 xorshift() {
  static u64 x = 88172645463325252ull;
  x = x ^ (x << 7);
  return x = x ^ (x >> 9);
}
template <typename T>
class pqasc : public priority_queue<T, vector<T>, greater<T>> {};
template <typename T>
class pqdesc : public priority_queue<T, vector<T>, less<T>> {};
template <typename T>
inline void amax(T &x, T y) {
  if (x < y) x = y;
}
template <typename T>
inline void amin(T &x, T y) {
  if (x > y) x = y;
}
template <typename T>
inline T exp(T x, i64 n, T e = 1) {
  T r = e;
  while (n > 0) {
    if (n & 1) r *= x;
    x *= x;
    n >>= 1;
  }
  return r;
}
template <typename T>
inline T bis(T ok, T ng, function<bool(T)> f, T eps = 1) {
  while (abs(ok - ng) > eps) {
    T mi = (ok + ng) / 2;
    (f(mi) ? ok : ng) = mi;
  }
  return ok;
}
template <typename T>
istream &operator>>(istream &is, vector<T> &v) {
  for (auto &x : v) is >> x;
  return is;
}
template <typename T>
ostream &operator<<(ostream &os, vector<T> &v) {
  for (int i = 0; i < int(v.size()); i++) {
    if (i) os << ' ';
    os << v[i];
  }
  return os;
}
template <typename T, typename S>
istream &operator>>(istream &is, pair<T, S> &p) {
  is >> p.first >> p.second;
  return is;
}
template <typename T, typename S>
ostream &operator<<(ostream &os, pair<T, S> &p) {
  os << p.first << ' ' << p.second;
  return os;
}
void solve();
int main() {
  ios::sync_with_stdio(0);
  cin.tie(0);
  cout << fixed << setprecision(16);
  solve();
  return 0;
}
int dir[] = {1, 0, -1, 0};
int e[401][401];
void solve() {
  int h, w;
  cin >> h >> w;
  int n = h * w;
  auto conv = [&](int x, int y) { return x * h + y; };
  vstr s(h);
  for (int i = 0; i < int(h); i++) cin >> s[i];
  for (int i = 0; i < int(n); i++)
    for (int j = 0; j < int(n); j++) e[i][j] = 1e9;
  for (int i = 0; i < int(h); i++)
    for (int j = 0; j < int(w); j++) {
      if (s[i][j] == '#') continue;
      for (int k = 0; k < int(4); k++) {
        int x = i + dir[k];
        int y = j + dir[k ^ 1];
        if (x < 0 || x >= h || y < 0 || y >= w) continue;
        if (s[x][y] == '#') continue;
        e[conv(i, j)][conv(x, y)] = 1;
      }
    }
  for (int k = 0; k < int(n); k++)
    for (int i = 0; i < int(n); i++)
      for (int j = 0; j < int(n); j++) amin(e[i][j], e[i][k] + e[k][j]);
  int ans = 0;
  for (int i = 0; i < int(n); i++)
    for (int j = 0; j < int(n); j++)
      if (e[i][j] != 1e9) amax(ans, e[i][j]);
  cout << ans << endl;
}
