#include <bits/stdc++.h>
using namespace std;
using ll = long long;
unsigned euclidean_gcd(unsigned a, unsigned b) {
  if (a < b) return euclidean_gcd(b, a);
  unsigned r;
  while ((r = a % b)) {
    a = b;
    b = r;
  }
  return b;
}
ll ll_gcd(ll a, ll b) {
  if (a < b) return ll_gcd(b, a);
  ll r;
  while ((r = a % b)) {
    a = b;
    b = r;
  }
  return b;
}
struct UnionFind {
  vector<ll> par;
  vector<ll> siz;
  UnionFind(ll sz_) : par(sz_), siz(sz_, 1LL) {
    for (ll i = 0; i < sz_; ++i) par[i] = i;
  }
  void init(ll sz_) {
    par.resize(sz_);
    siz.assign(sz_, 1LL);
    for (ll i = 0; i < sz_; ++i) par[i] = i;
  }
  ll root(ll x) {
    while (par[x] != x) {
      x = par[x] = par[par[x]];
    }
    return x;
  }
  bool merge(ll x, ll y) {
    x = root(x);
    y = root(y);
    if (x == y) return false;
    if (siz[x] < siz[y]) swap(x, y);
    siz[x] += siz[y];
    par[y] = x;
    return true;
  }
  bool issame(ll x, ll y) { return root(x) == root(y); }
  ll size(ll x) { return siz[root(x)]; }
};
long long modpow(long long a, long long n, long long mod) {
  long long res = 1;
  while (n > 0) {
    if (n & 1) res = res * a % mod;
    a = a * a % mod;
    n >>= 1;
  }
  return res;
}
long long modinv(long long a, long long mod) { return modpow(a, mod - 2, mod); }
vector<int> tpsort(vector<vector<int>>& G) {
  int V = G.size();
  vector<int> sorted_vertices;
  queue<int> que;
  vector<int> indegree(V);
  for (int i = 0; i < V; i++) {
    for (int j = 0; j < G[i].size(); j++) {
      indegree[G[i][j]]++;
    }
  }
  for (int i = 0; i < V; i++) {
    if (indegree[i] == 0) {
      que.push(i);
    }
  }
  while (que.empty() == false) {
    int v = que.front();
    que.pop();
    for (int i = 0; i < G[v].size(); i++) {
      int u = G[v][i];
      indegree[u] -= 1;
      if (indegree[u] == 0) que.push(u);
    }
    sorted_vertices.push_back(v);
  }
  return sorted_vertices;
}
struct Point {
  double x;
  double y;
};
struct LineSegment {
  Point start;
  Point end;
};
double tenkyori(const LineSegment& line, const Point& point) {
  double x0 = point.x, y0 = point.y;
  double x1 = line.start.x, y1 = line.start.y;
  double x2 = line.end.x, y2 = line.end.y;
  double a = x2 - x1;
  double b = y2 - y1;
  double a2 = a * a;
  double b2 = b * b;
  double r2 = a2 + b2;
  double tt = -(a * (x1 - x0) + b * (y1 - y0));
  if (tt < 0)
    return sqrt((x1 - x0) * (x1 - x0) + (y1 - y0) * (y1 - y0));
  else if (tt > r2)
    return sqrt((x2 - x0) * (x2 - x0) + (y2 - y0) * (y2 - y0));
  double f1 = a * (y1 - y0) - b * (x1 - x0);
  return sqrt((f1 * f1) / r2);
}
void dfs1(vector<vector<ll>>& z, ll k, ll oya, ll& ans, vector<ll>& b) {
  for (auto m : z[k]) {
    if (m != oya) dfs1(z, m, k, ans, b);
  }
  vector<ll> s;
  for (auto m : z[k]) {
    if (m != oya) s.push_back(b[m]);
  }
  ll m = b.size() - 1;
  for (auto d : s) {
    m -= d;
  }
  b[k] = b.size() - m;
  if (m != 0) s.push_back(m);
  ll a = modinv(2, 1000000007);
  for (auto d : s) {
    a += 1000000007 - modinv(modpow(2, b.size() - d, 1000000007), 1000000007);
  }
  a += modinv(modpow(2, b.size(), 1000000007), 1000000007) * (z[k].size() - 1);
  ans += a;
  ans %= 1000000007;
  return;
}
ll merge_cnt(vector<int>& a) {
  int n = a.size();
  if (n <= 1) {
    return 0;
  }
  ll cnt = 0;
  vector<int> b(a.begin(), a.begin() + n / 2);
  vector<int> c(a.begin() + n / 2, a.end());
  cnt += merge_cnt(b);
  cnt += merge_cnt(c);
  int ai = 0, bi = 0, ci = 0;
  while (ai < n) {
    if (bi < b.size() && (ci == c.size() || b[bi] <= c[ci])) {
      a[ai++] = b[bi++];
    } else {
      cnt += n / 2 - bi;
      a[ai++] = c[ci++];
    }
  }
  return cnt;
}
int main() {
  ll h, w;
  cin >> h >> w;
  vector<string> z(h);
  for (int i = 0; i < h; i++) {
    cin >> z[i];
  }
  ll ans = 0;
  for (int i = 0; i < h; i++) {
    for (int j = 0; j < w; j++) {
      if (z[i][j] == '#') continue;
      vector<vector<bool>> bol(h, vector<bool>(w, false));
      queue<pair<pair<ll, ll>, ll>> q;
      q.push(make_pair(make_pair(i, j), 0));
      ll now = 0;
      while (!q.empty()) {
        ll x = q.front().first.first;
        ll y = q.front().first.second;
        ll s = q.front().second;
        now = s;
        bol[x][y] = true;
        if (x != 0 && z[x - 1][y] == '.' && bol[x - 1][y] == false) {
          q.push(make_pair(make_pair(x - 1, y), s + 1));
        }
        if (y != 0 && z[x][y - 1] == '.' && bol[x][y - 1] == false) {
          q.push(make_pair(make_pair(x, y - 1), s + 1));
        }
        if (x != h - 1 && z[x + 1][y] == '.' && bol[x + 1][y] == false) {
          q.push(make_pair(make_pair(x + 1, y), s + 1));
        }
        if (y != w - 1 && z[x][y + 1] == '.' && bol[x][y + 1] == false) {
          q.push(make_pair(make_pair(x, y + 1), s + 1));
        }
        ans = max(ans, now);
        q.pop();
      }
    }
  }
  cout << ans << endl;
}
