#include <bits/stdc++.h>
using namespace std;
template <class C>
void min_self(C &a, C b) {
  a = min(a, b);
}
template <class C>
void max_self(C &a, C b) {
  a = max(a, b);
}
long long mod(long long n, long long m = 1000000007) {
  n %= m, n += m, n %= m;
  return n;
}
mt19937 rng(chrono::steady_clock::now().time_since_epoch().count());
struct custom_hash {
  static uint64_t splitmix64(uint64_t x) {
    x += 0x9e3779b97f4a7c15;
    x = (x ^ (x >> 30)) * 0xbf58476d1ce4e5b9;
    x = (x ^ (x >> 27)) * 0x94d049bb133111eb;
    return x ^ (x >> 31);
  }
  size_t operator()(uint64_t x) const {
    static const uint64_t FIXED_RANDOM =
        chrono::steady_clock::now().time_since_epoch().count();
    return splitmix64(x + FIXED_RANDOM);
  }
};
unordered_map<long long, int, custom_hash> safe_map;
const int MAXN = 1e5 + 5;
const int LOGN = 21;
const long long INF = 1e14;
int dx[] = {1, 0, -1, 0};
int dy[] = {0, 1, 0, -1};
char grid[25][25];
int h, w;
bool inside(int x, int y) {
  if (x < 0 || x >= h || y < 0 || y >= w) return 0;
  return 1;
}
int bfs(int i, int j) {
  queue<tuple<int, int, int>> q;
  tuple<int, int, int> t = {i, j, 0};
  q.push(t);
  bool vis[25][25] = {};
  int ans = 0;
  while (!q.empty()) {
    int x, y, step;
    tie(x, y, step) = q.front();
    q.pop();
    if (vis[x][y]) continue;
    vis[x][y] = 1;
    ans = max(ans, step);
    for (int i = 0; i < 4; i++) {
      int nx = x + dx[i], ny = y + dy[i];
      if (inside(nx, ny) && grid[nx][ny] == '.') {
        t = {nx, ny, step + 1};
        q.push(t);
      }
    }
  }
  return ans;
}
int main() {
  ios_base::sync_with_stdio(0);
  cin.tie(0);
  cout.tie(0);
  cin >> h >> w;
  for (int i = 0; i < h; i++) {
    for (int j = 0; j < w; j++) {
      cin >> grid[i][j];
    }
  }
  int mx = 0;
  for (int i = 0; i < h; i++) {
    for (int j = 0; j < w; j++) {
      if (grid[i][j] == '#') continue;
      mx = max(mx, bfs(i, j));
    }
  }
  cout << mx, cout << "\n";
  cerr << "\nTime elapsed: " << 1000 * clock() / CLOCKS_PER_SEC << "ms\n";
  return 0;
}
