#include <bits/stdc++.h>
#pragma GCC optimize("Ofast")
using namespace std;
using ll = long long;
using VI = vector<ll>;
using VV = vector<VI>;
using VS = vector<string>;
using PII = pair<ll, ll>;
template <typename A, typename B>
string to_string(pair<A, B> p);
template <typename A, typename B, typename C>
string to_string(tuple<A, B, C> p);
template <typename A, typename B, typename C, typename D>
string to_string(tuple<A, B, C, D> p);
string to_string(const string &s) { return '"' + s + '"'; }
string to_string(const char *s) { return to_string((string)s); }
string to_string(bool b) { return (b ? "true" : "false"); }
string to_string(vector<bool> v) {
  bool first = true;
  string res = "{";
  for (int i = 0; i < static_cast<int>(v.size()); i++) {
    if (!first) {
      res += ", ";
    }
    first = false;
    res += to_string(v[i]);
  }
  res += "}";
  return res;
}
template <size_t N>
string to_string(bitset<N> v) {
  string res = "";
  for (size_t i = 0; i < N; i++) {
    res += static_cast<char>('0' + v[i]);
  }
  return res;
}
template <typename A>
string to_string(A v) {
  bool first = true;
  string res = "{";
  for (const auto &x : v) {
    if (!first) {
      res += ", ";
    }
    first = false;
    res += to_string(x);
  }
  res += "}";
  return res;
}
template <typename A, typename B>
string to_string(pair<A, B> p) {
  return "(" + to_string(p.first) + ", " + to_string(p.second) + ")";
}
template <typename A, typename B, typename C>
string to_string(tuple<A, B, C> p) {
  return "(" + to_string(get<0>(p)) + ", " + to_string(get<1>(p)) + ", " +
         to_string(get<2>(p)) + ")";
}
template <typename A, typename B, typename C, typename D>
string to_string(tuple<A, B, C, D> p) {
  return "(" + to_string(get<0>(p)) + ", " + to_string(get<1>(p)) + ", " +
         to_string(get<2>(p)) + ", " + to_string(get<3>(p)) + ")";
}
void debug_out() { cerr << '\n'; }
template <typename Head, typename... Tail>
void debug_out(Head H, Tail... T) {
  cerr << " " << to_string(H);
  debug_out(T...);
}
template <class T>
bool chmax(T &a, const T &b) {
  if (a < b) {
    a = b;
    return 1;
  }
  return 0;
}
template <class T>
bool chmin(T &a, const T &b) {
  if (b < a) {
    a = b;
    return 1;
  }
  return 0;
}
void no() {
  cout << ("NO") << '\n';
  exit(0);
}
void yes() {
  cout << ("YES") << '\n';
  exit(0);
}
const ll mod = 1e9 + 7;
const ll inf = 1e18;
const double PI = acos(-1);
ll d[30][30];
void reset_d() {
  for (ll i = (0); i < (30); ++i) {
    for (ll j = (0); j < (30); ++j) {
      d[i][j] = inf;
    }
  }
}
int dx[4] = {-1, 0, 1, 0};
int dy[4] = {0, 1, 0, -1};
ll H, W;
bool in_range(ll i, ll j) {
  if (0 <= i && i < H && 0 <= j && j < W) {
    return true;
  } else {
    return false;
  }
}
int main() {
  cin.tie(0);
  ios::sync_with_stdio(false);
  cin >> H >> W;
  vector<string> M(H);
  for (ll i = (0); i < (H); ++i) {
    cin >> M[i];
  }
  PII start;
  for (ll i = (0); i < (H); ++i) {
    for (ll j = (0); j < (W); ++j) {
      if (M[i][j] == '.') {
        start = make_pair(i, j);
      }
    }
  }
  reset_d();
  d[start.first][start.second] = 0;
  queue<PII> que;
  que.push(start);
  while (!que.empty()) {
    auto pa = que.front();
    que.pop();
    ll y = pa.first;
    ll x = pa.second;
    ll dist = d[y][x];
    for (ll i = (0); i < (4); ++i) {
      ll ty = y + dy[i];
      ll tx = x + dx[i];
      if (in_range(ty, tx) && M[ty][tx] == '.' && d[ty][tx] == inf) {
        que.push(make_pair(ty, tx));
        d[ty][tx] = dist + 1;
      }
    }
  }
  ll max_dist = 0;
  PII max_pair;
  for (ll i = (0); i < (H); ++i) {
    for (ll j = (0); j < (W); ++j) {
      if (d[i][j] == inf) continue;
      if (max_dist < d[i][j]) {
        max_pair = make_pair(i, j);
        max_dist = d[i][j];
      }
    }
  }
  42;
  42;
  reset_d();
  d[max_pair.first][max_pair.second] = 0;
  que.push(make_pair(max_pair.first, max_pair.second));
  while (!que.empty()) {
    auto pa = que.front();
    que.pop();
    ll y = pa.first;
    ll x = pa.second;
    ll dist = d[y][x];
    for (ll i = (0); i < (4); ++i) {
      ll ty = y + dy[i];
      ll tx = x + dx[i];
      if (in_range(ty, tx) && M[ty][tx] == '.' && d[ty][tx] == inf) {
        que.push(make_pair(ty, tx));
        d[ty][tx] = dist + 1;
      }
    }
  }
  ll ma = 0;
  for (ll i = (0); i < (H); ++i) {
    for (ll j = (0); j < (W); ++j) {
      if (d[i][j] == inf) continue;
      chmax(ma, d[i][j]);
    }
  }
  cout << (ma) << '\n';
  return 0;
}
