#include <bits/stdc++.h>
using namespace std;
using lint = long long int;
using ulint = unsigned long long int;
using pint = pair<int, int>;
using plint = pair<lint, lint>;
struct fast_ios { fast_ios(){ cin.tie(0); ios::sync_with_stdio(false); cout << fixed << setprecision(20); }; } fast_ios_;
#define ALL(x) (x).begin(), (x).end()
#define SZ(x) ((lint)(x).size())
#define POW2(n) (1LL << (n))
#define FOR(i, begin, end) for(int i=(begin),i##_end_=(end);i<i##_end_;i++)
#define IFOR(i, begin, end) for(int i=(end)-1,i##_begin_=(begin);i>=i##_begin_;i--)
#define REP(i, n) FOR(i,0,n)
#define IREP(i, n) IFOR(i,0,n)
template<typename T> istream &operator>>(istream &is, vector<T> &vec){ for (auto &v : vec) is >> v; return is; }
template<typename T> ostream &operator<<(ostream &os, const vector<T> &vec){ os << "["; for (auto v : vec) os << v << ","; os << "]"; return os; }
template<typename T> ostream &operator<<(ostream &os, const deque<T> &vec){ os << "deq["; for (auto v : vec) os << v << ","; os << "]"; return os; }
template<typename T> ostream &operator<<(ostream &os, const set<T> &vec){ os << "{"; for (auto v : vec) os << v << ","; os << "}"; return os; }
template<typename T> ostream &operator<<(ostream &os, const unordered_set<T> &vec){ os << "{"; for (auto v : vec) os << v << ","; os << "}"; return os; }
template<typename T> ostream &operator<<(ostream &os, const multiset<T> &vec){ os << "{"; for (auto v : vec) os << v << ","; os << "}"; return os; }
template<typename T> ostream &operator<<(ostream &os, const unordered_multiset<T> &vec){ os << "{"; for (auto v : vec) os << v << ","; os << "}"; return os; }
template<typename T1, typename T2> ostream &operator<<(ostream &os, const pair<T1, T2> &pa){ os << "(" << pa.first << "," << pa.second << ")"; return os; }
template<typename TK, typename TV> ostream &operator<<(ostream &os, const map<TK, TV> &mp){ os << "{"; for (auto v : mp) os << v.first << "=>" << v.second << ","; os << "}"; return os; }
template<typename TK, typename TV> ostream &operator<<(ostream &os, const unordered_map<TK, TV> &mp){ os << "{"; for (auto v : mp) os << v.first << "=>" << v.second << ","; os << "}"; return os; }
template<typename T> void ndarray(vector<T> &vec, int len) { vec.resize(len); }
template<typename T, typename... Args> void ndarray(vector<T> &vec, int len, Args... args) { vec.resize(len); for (auto &v : vec) ndarray(v, args...); }
template<typename T> bool mjjj(T &m, const T q) { if (m < q) {m = q; return true;} else return false; }
template<typename T> bool mmin(T &m, const T q) { if (m > q) {m = q; return true;} else return false; }
template<typename T1, typename T2> pair<T1, T2> operator+(const pair<T1, T2> &l, const pair<T1, T2> &r) { return make_pair(l.first + r.first, l.second + r.second); }
template<typename T1, typename T2> pair<T1, T2> operator-(const pair<T1, T2> &l, const pair<T1, T2> &r) { return make_pair(l.first - r.first, l.second - r.second); }
#define dbg(x) cerr << #x << " = " << (x) << " (L" << __LINE__ << ") " << __FILE__ << endl;
/*
#include <ext/pb_ds/assoc_container.hpp>
#include <ext/pb_ds/tree_policy.hpp>
#include <ext/pb_ds/tag_and_trait.hpp>
using namespace __gnu_pbds; // find_by_order(), order_of_key()
template<typename TK> using pbds_set = tree<TK, null_type, less<TK>, rb_tree_tag, tree_order_statistics_node_update>;
template<typename TK, typename TV> using pbds_map = tree<TK, TV, less<TK>, rb_tree_tag, tree_order_statistics_node_update>;
*/

template< typename T >
struct edge {
int src, to;
T cost;
//edge(int to) : src(-1),to(to),cost(-1){}
edge(int to, T cost) : src(-1), to(to), cost(cost) {}
edge(int src, int to, T cost) : src(src), to(to), cost(cost) {}
edge &operator=(const int &x) {
to = x;
return *this;
}
operator int() const { return to; }
};
template< typename T >
using Edges = vector< edge< T > >;
template< typename T >
using WeightedGraph = vector< Edges< T > >;
using UnWeightedGraph = vector< vector< lint > >;
template< typename T >
using Matrix = vector< vector< T > >;
#define INF 1e+9
#define MAX_V 100000

// <最短距離, 頂点の番号>
using P = pair<lint, lint>;

lint V;
vector<edge<lint>> G[MAX_V];
lint d[MAX_V];

void dijkstra(lint s) {
    priority_queue<P, vector<P>, greater<P> > que;
    fill(d, d+V, INF);
    d[s] = 0;
    que.push(P(0, s));

    while (!que.empty()) {
        P p = que.top();
        que.pop();
        lint v = p.second;
        if (d[v] < p.first) continue;

        for (lint i=0; i<G[v].size(); ++i) {
            edge<lint> e = G[v][i];
            if (d[e.to] > d[v] + e.cost) {
                d[e.to] = d[v] + e.cost;
                que.push(P(d[e.to], e.to));
            }
        }
    }
}

int main()
{
  ios::sync_with_stdio(false);
	cin.tie(nullptr);
	cout.tie(nullptr);
  lint h,w;
  vector<vector<lint>> rotate(4,vector<lint>(2));
  rotate[0][0] = 1;
  rotate[0][1] = 0;
  rotate[1][0] = 0;
  rotate[1][1] = 1;
  rotate[2][0] = -1;
  rotate[2][1] = 0;
  rotate[3][0] = 0;
  rotate[3][1] = -1;
  cin >> h >> w;
  vector<vector<char>> s(h,vector<char>(w));
  REP(i,h){
    REP(j,w){
      cin >> s[i][j];
    }
  }
  REP(i,h){
    REP(j,w){
      REP(k,4){
          if((i + rotate[k][1]) >= 0 && (j + rotate[k][0]) >= 0 
          && (i + rotate[k][1]) < h && (j + rotate[k][0]) < w ){
            if(s[i + rotate[k][1]][j + rotate[k][0]] == '#'){
             G[i*w+j].push_back(edge<lint>((i + rotate[k][1])*w+(j + rotate[k][0]),INF));
            }
            else{
             G[i*w+j].push_back(edge<lint>((i + rotate[k][1])*w+(j + rotate[k][0]),1));
            }
          }
      }
    }
  }
  lint ans = 0;
  V = h*w;
  REP(i,h*w){
    if(s[i/w][i%w] == '#')continue;
    dijkstra(i);
    REP(j,h*w){
      if(s[j/w][j%w] == '#')continue;
      ans = max(ans,d[j]);
    }
  }
  cout << ans << endl;
}

Measured memory_bytes: 2688