#include <bits/stdc++.h>
#pragma GCC optimize("Ofast")
using namespace std;
template <typename T>
T sqr(T a) {
  return a * a;
}
template <typename T>
int sign(T a) {
  if (a == 0) return 0;
  return (a > 0 ? 1 : -1);
}
template <typename T1, typename T2>
T2 sum_mod(T1 x, T2 MOD) {
  if (x < 0) x += MOD;
  if (x >= MOD) x -= MOD;
  return x;
}
template <typename T1, typename T2>
T2 mul_mod(T1 x, T2 MOD) {
  if (x < 0) return (abs(x) % MOD == 0 ? 0 : MOD - abs(x) % MOD);
  return x % MOD;
}
template <typename T>
bool uax(T& a, const T b) {
  if (a < b) {
    a = b;
    return true;
  }
  return false;
}
template <typename T>
bool uin(T& a, const T b) {
  if (a > b) {
    a = b;
    return true;
  }
  return false;
}
template <typename T1, typename T2>
istream& operator>>(istream& in, pair<T1, T2>& p) {
  in >> p.first >> p.second;
  return in;
}
template <typename T1, typename T2>
ostream& operator<<(ostream& out, pair<T1, T2>& p) {
  out << p.first << ' ' << p.second;
  return out;
}
mt19937_64 gen_rand;
const int N = (int)2e5 + 7;
const int LOG = 20;
const int N_ = (int)8e2 + 7;
const int MAXN = (int)2e6 + 7;
const int MOD = (int)1e9 + 7;
const int INF = (int)2e9 + 7;
const int CONST = 300;
const long long LINF = (int64_t)2e15;
const long double PI = 3.1415926535897932384626433832795;
const long double EPS = 1e-6;
int n, m, d[22][22];
char f[22][22];
int bfs(int x, int y) {
  for (int i = 0; i < 22; i++) fill(d[i], d[i] + 22, INF);
  d[x][y] = 0;
  queue<pair<int, int>> q;
  q.push(make_pair(x, y));
  int res = 0;
  int h[4] = {-1, 0, 0, 1}, v[4] = {0, -1, 1, 0};
  while (!q.empty()) {
    int x, y;
    tie(x, y) = q.front();
    q.pop();
    uax(res, d[x][y]);
    for (int i = 0; i < 4; i++) {
      int nx = x + h[i], ny = y + v[i];
      if (f[nx][ny] == '.' && uin(d[nx][ny], d[x][y] + 1))
        q.push(make_pair(nx, ny));
    }
  }
  return res;
}
int main() {
  double start = clock();
  cout << fixed << setprecision(20);
  ios_base::sync_with_stdio(0);
  cin.tie(0);
  cout.tie(0);
  gen_rand.seed(time(0));
  cin >> n >> m;
  for (int i = 0; i < 22; i++) fill(f[i], f[i] + 22, '#');
  for (int i = 1; i <= n; i++) {
    for (int j = 1; j <= m; j++) cin >> f[i][j];
  }
  int ans = 0;
  for (int i = 1; i <= n; i++) {
    for (int j = 1; j <= m; j++) uax(ans, bfs(i, j));
  }
  cout << ans;
}
