#include <bits/stdc++.h>
using namespace std;
long long day[12] = {31, 28, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31};
long long dx[] = {0, 1, 0, -1};
long long dy[] = {1, 0, -1, 0};
const long long MOD = 1000000007LL;
bool isupper(char c) {
  if ('A' <= c && c <= 'Z') return 1;
  return 0;
}
bool islower(char c) {
  if ('a' <= c && c <= 'z') return 1;
  return 0;
}
bool iskaibun(string s) {
  for (long long i = 0; i < s.size() / 2; i++)
    if (s[i] != s[s.size() - i - 1]) return 0;
  return 1;
}
bool isnumber(char c) { return ('0' <= c && c <= '9'); }
bool isalpha(char c) { return (isupper(c) || islower(c)); }
template <typename T>
void print(vector<T> v) {
  for (long long i = 0; i < v.size(); i++) {
    if (i) cout << " ";
    cout << v[i];
  }
  cout << endl;
}
long long INF = 1e18;
template <typename T>
void printendl(vector<T> v) {
  for (auto date : v) cout << date << endl;
}
template <typename T>
void printvv(vector<vector<T>> v) {
  for (long long i = 0; i < v.size(); i++) {
    for (long long j = 0; j < v[i].size(); j++) {
      if (j) cout << " ";
      cout << v[i][j];
    }
    cout << endl;
  }
}
struct Point {
  long long x, y, num;
  double e_dis(Point other) {
    return sqrt((x - other.x) * (x - other.x) + (y - other.y) * (y - other.y));
  }
};
long long gcd(long long a, long long b) {
  if (b == 0)
    return a;
  else
    return gcd(b, a % b);
}
bool isPrime(long long x) {
  if (x == 1) return 0;
  if (x == 2) return 1;
  if (x % 2 == 0) return 0;
  for (long long i = 3; i * i <= x; i++)
    if (x % i == 0) return 0;
  return 1;
}
long long d(long long x) {
  long long ret = 0;
  while (x > 0) {
    ret++;
    x /= 10;
  }
  return ret;
}
void dfs(vector<long long>& ans, vector<vector<long long>>& v,
         long long max_edge, long long parent, long long idx) {
  long long color_idx = 1;
  for (long long i = (long long)0; i < (long long)v[idx].size(); i++) {
    if (v[idx][i] == parent) continue;
    while (color_idx == ans[idx]) color_idx++;
    ans[v[idx][i]] = color_idx;
    dfs(ans, v, max_edge, idx, v[idx][i]);
    color_idx++;
  }
  return;
}
vector<Point> bfspoint(long long x, long long y, vector<string>& v, long long h,
                       long long w) {
  queue<Point> que;
  Point init = {x, y, 0};
  que.push(init);
  vector<vector<long long>> visited(h, vector<long long>(w, -1));
  visited[init.y][init.x] = 0;
  while (!que.empty()) {
    Point now = que.front();
    que.pop();
    for (long long i = (long long)0; i < (long long)4; i++) {
      long long nx = dx[i] + now.x;
      long long ny = dy[i] + now.y;
      if (0 <= nx && 0 <= ny && nx < w && ny < h && v[ny][nx] != '#' &&
          visited[ny][nx] == -1) {
        Point next = {nx, ny, now.num + 1};
        que.push(next);
        visited[ny][nx] = now.num + 1;
      }
    }
  }
  long long ma = -1;
  vector<Point> ret;
  for (long long i = (long long)0; i < (long long)h; i++) {
    for (long long j = (long long)0; j < (long long)w; j++) {
      ma = max(ma, visited[i][j]);
    }
  }
  for (long long i = (long long)0; i < (long long)h; i++) {
    for (long long j = (long long)0; j < (long long)w; j++) {
      if (ma == visited[i][j]) {
        ret.push_back(Point{j, i});
      }
    }
  }
  return ret;
}
long long bfsint(long long x, long long y, vector<string>& v, long long h,
                 long long w) {
  queue<Point> que;
  Point init = {x, y, 0};
  que.push(init);
  vector<vector<long long>> visited(h, vector<long long>(w, -1));
  visited[init.y][init.x] = 0;
  while (!que.empty()) {
    Point now = que.front();
    que.pop();
    for (long long i = (long long)0; i < (long long)4; i++) {
      long long nx = dx[i] + now.x;
      long long ny = dy[i] + now.y;
      if (0 <= nx && 0 <= ny && nx < w && ny < h && v[ny][nx] != '#' &&
          visited[ny][nx] == -1) {
        Point next = {nx, ny, now.num + 1};
        que.push(next);
        visited[ny][nx] = now.num + 1;
      }
    }
  }
  long long ret = -1;
  for (long long i = (long long)0; i < (long long)h; i++) {
    for (long long j = (long long)0; j < (long long)w; j++) {
      ret = max(ret, visited[i][j]);
    }
  }
  return ret;
}
signed main() {
  long long h, w;
  cin >> h >> w;
  vector<string> v(h);
  for (long long i = (long long)0; i < (long long)h; i++) cin >> v[i];
  for (long long i = (long long)0; i < (long long)h; i++) {
    for (long long j = (long long)0; j < (long long)w; j++) {
      if (v[i][j] == '.') {
        vector<Point> something = bfspoint(j, i, v, h, w);
        long long ans = 0;
        for (long long k = (long long)0; k < (long long)something.size(); k++) {
          ans = max(ans, bfsint(something[k].x, something[k].y, v, h, w));
        }
        cout << ans << endl;
        return 0;
      }
    }
  }
}
