#include <bits/stdc++.h>
using namespace std;
using LLONG = long long;
const LLONG MOD = 1000000007;
class Graph {
 private:
  int m_nodeNum;
  vector<vector<int>> m_adList;
  vector<vector<int>> m_connectedComponents;
  vector<int> m_connectedCompunentIndex;

 public:
  Graph(const int N) : m_nodeNum(N) {
    m_adList = vector<vector<int>>(N);
    m_connectedCompunentIndex = vector<int>(N, -1);
  }
  void Connect(const int v, const int u) {
    m_adList[v].push_back(u);
    m_adList[u].push_back(v);
  }
  void ComputeConnectedComponents() {
    vector<bool> hasNotVisited(m_nodeNum, true);
    stack<int> stack;
    int ccIndex = 0;
    vector<int> cc;
    for (int v = 0; v < m_nodeNum; ++v) {
      if (hasNotVisited[v]) {
        stack.push(v);
        cc.clear();
        while (!stack.empty()) {
          int u = stack.top();
          stack.pop();
          hasNotVisited[u] = false;
          m_connectedCompunentIndex[u] = ccIndex;
          cc.push_back(u);
          for (const int n : m_adList[u]) {
            if (hasNotVisited[n]) {
              hasNotVisited[n] = false;
              stack.push(n);
            }
          }
        }
        m_connectedComponents.push_back(cc);
        ++ccIndex;
      }
    }
  }
  const int GetConnectedComponentSize(const int v) {
    int index = m_connectedCompunentIndex[v];
    return static_cast<int>(m_connectedComponents[index].size());
  }
  const int GetConnectedComponentIndex(const int v) {
    return m_connectedCompunentIndex[v];
  }
  const int Distance(const int u, const int v) {
    vector<int> distances(m_nodeNum, m_nodeNum + 1);
    distances[u] = 0;
    deque<int> Q{u};
    while (!Q.empty()) {
      const int key = Q.back();
      Q.pop_back();
      for (const int n : m_adList[key]) {
        int tmpDist = distances[key] + 1;
        if (tmpDist < distances[n]) {
          distances[n] = tmpDist;
          Q.push_back(n);
        }
      }
    }
    return distances[v];
  }
};
int main() {
  int H, W;
  cin >> H >> W;
  vector<vector<bool>> S(H, vector<bool>(W));
  for (int r = 0; r < H; ++r) {
    for (int c = 0; c < W; ++c) {
      char ch;
      cin >> ch;
      S[r][c] = ch == '.';
    }
  }
  Graph graph(H * W);
  vector<int> roads;
  for (int r = 0; r < H; ++r) {
    for (int c = 0; c < W; ++c) {
      int ID = r * W + c;
      if (S[r][c]) {
        roads.push_back(ID);
        if (c < W - 1 && S[r][c + 1]) {
          graph.Connect(ID, r * W + c + 1);
        }
        if (r < H - 1 && S[r + 1][c]) {
          graph.Connect(ID, (r + 1) * W + c);
        }
      }
    }
  }
  int ans = 0;
  int num = roads.size();
  for (int i = 0; i < num; ++i) {
    for (int j = i + 1; j < num; ++j) {
      int tmp = graph.Distance(roads[i], roads[j]);
      ans = max(ans, tmp);
    }
  }
  cout << ans << endl;
}
