#include <bits/stdc++.h>
using namespace std;
template <typename T>
inline bool uax(T &x, T y) {
  return (y > x) ? x = y, true : false;
}
template <typename T>
inline bool uin(T &x, T y) {
  return (y < x) ? x = y, true : false;
}
string to_string(char c) { return "'" + string(1, c) + "'"; }
string to_string(string s) { return '"' + s + '"'; }
string to_string(const char *s) { return to_string((string)s); }
template <typename A>
string to_string(A);
template <typename A, typename B>
string to_string(pair<A, B> p) {
  return "(" + to_string(p.first) + ": " + to_string(p.second) + ")";
}
template <typename A>
string to_string(A v) {
  bool f = false;
  string r = "{";
  for (auto x : v) {
    if (f) r += ", ";
    r += to_string(x);
    f = true;
  }
  return r += "}";
}
template <typename A>
string to_string(vector<vector<A>> v) {
  string r;
  for (auto x : v) r += "\n" + to_string(x);
  return r;
}
void err(istream_iterator<string>) { cerr << endl; }
template <typename T, typename... Args>
void err(istream_iterator<string> it, T a, Args... args) {
  cerr << *it << " =: " << to_string(a) << "; ";
  err(++it, args...);
}
template <typename T>
void kek(T ans) {
  cout << ans << endl;
  exit(0);
}
int const MOD = 1e9 + 7;
long long const INF = 1e18 + 42;
const int N = 22;
int n, m;
int a[N][N], b[N][N];
queue<pair<int, int>> q;
int dx[] = {1, -1, 0, 0};
int dy[] = {0, 0, 1, -1};
void go(int x, int y, int val) {
  if (x < 0 or x >= n or y < 0 or y >= m or a[x][y] or b[x][y] != -1) return;
  b[x][y] = val;
  q.push({x, y});
}
int solve(int x, int y) {
  if (a[x][y]) return -1;
  for (int i = 0; i < n; ++i) {
    for (int j = 0; j < m; ++j) {
      b[i][j] = -1;
    }
  }
  q = {};
  q.push({x, y});
  b[x][y] = 0;
  while (!q.empty()) {
    int u = q.front().first, v = q.front().second;
    q.pop();
    for (int i = 0; i < 4; ++i) {
      go(u + dx[i], v + dy[i], 1 + b[u][v]);
    }
  }
  int res = -1;
  for (int i = 0; i < n; ++i) {
    for (int j = 0; j < m; ++j) {
      uax(res, b[i][j]);
    }
  }
  return res;
}
int32_t main() {
  ios_base::sync_with_stdio(false);
  cin.tie(nullptr);
  cin >> n >> m;
  for (int i = 0; i < n; ++i) {
    string s;
    cin >> s;
    for (int j = 0; j < m; ++j) {
      a[i][j] = s[j] == '#';
    }
  }
  int ans = 0;
  for (int i = 0; i < n; ++i) {
    for (int j = 0; j < m; ++j) {
      uax(ans, solve(i, j));
    }
  }
  kek(ans);
}
