#include <cassert>
#include <cctype>
#include <cerrno>
#include <cfloat>
#include <ciso646>
#include <climits>
#include <clocale>
#include <cmath>
#include <csetjmp>
#include <csignal>
#include <cstdarg>
#include <cstddef>
#include <cstdio>
#include <cstdlib>
#include <cstring>
#include <ctime>
#include <cwchar>
#include <cwctype>
#include <ccomplex>
#include <cfenv>
#include <cinttypes>
#include <cstdbool>
#include <cstdint>
#include <ctgmath>
#include <algorithm>
#include <bitset>
#include <complex>
#include <deque>
#include <exception>
#include <fstream>
#include <functional>
#include <iomanip>
#include <ios>
#include <iosfwd>
#include <iostream>
#include <istream>
#include <iterator>
#include <limits>
#include <list>
#include <locale>
#include <map>
#include <memory>
#include <new>
#include <numeric>
#include <ostream>
#include <queue>
#include <set>
#include <sstream>
#include <stack>
#include <stdexcept>
#include <streambuf>
#include <string>
#include <typeinfo>
#include <utility>
#include <valarray>
#include <vector>
#include <array>
#include <atomic>
#include <chrono>
#include <codecvt>
#include <condition_variable>
#include <forward_list>
#include <future>
#include <initializer_list>
#include <mutex>
#include <random>
#include <ratio>
#include <regex>
#include <scoped_allocator>
#include <system_error>
#include <thread>
#include <tuple>
#include <typeindex>
#include <type_traits>
#include <unordered_map>
#include <unordered_set>
#define rep(i, n) for(int i = 0; i < (int)(n); i++)
using namespace std;
typedef int64_t ll;
typedef long double ld;
const ll INF=(1LL<<62);
const double inf=pow(10,10);
const ld pi=acosl(1);
const ll mod = 1000000007;
const int dx[4]={0,1,0,-1};
const int dy[4]={1,0,-1,0};
const int ddx[8]={1,0,-1,-1,-1,0,1,1};
const int ddy[8]={1,1,1,0,-1,-1,-1,0};
template< typename T >
struct edge {
  int src, to;
  T cost;

  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< int > >;
template< typename T >
using Matrix = vector< vector< T > >;
template< typename T >
void warshall_floyd(Matrix< T > &g, T INF) {
  for(int k = 0; k < g.size(); k++) {
    for(int i = 0; i < g.size(); i++) {
      for(int j = 0; j < g.size(); j++) {
        if(g[i][k] == INF || g[k][j] == INF) continue;
        g[i][j] = min(g[i][j], g[i][k] + g[k][j]);
      }
    }
  }
}
template< typename T >
vector< T > dijkstra(WeightedGraph< T > &g, int s) {
  const auto INF = numeric_limits< T >::max();
  vector< T > dist(g.size(), INF);

  using Pi = pair< T, int >;
  priority_queue< Pi, vector< Pi >, greater< Pi > > que;
  dist[s] = 0;
  que.emplace(dist[s], s);
  while(!que.empty()) {
    T cost;
    int idx;
    tie(cost, idx) = que.top();
    que.pop();
    if(dist[idx] < cost) continue;
    for(auto &e : g[idx]) {
      auto next_cost = cost + e.cost;
      if(dist[e.to] <= next_cost) continue;
      dist[e.to] = next_cost;
      que.emplace(dist[e.to], e.to);
    }
  }
  return dist;
}


int main(){
	ll H,W;cin>>H>>W;
	char map[H][W];
	rep(i,H)rep(j,W)cin>>map[i][j];
	
	//make graph
	WeightedGraph<ll>G(H*W);
	rep(i,H)rep(j,W){
		if(map[i][j]!='.')continue;
		rep(k,2){
			ll nextH=i+dx[k];
			ll nextW=j+dy[k];
			if(nextH<0||nextH>=H&&nextW<0||nextW>=W)continue;
			if(map[nextH][nextW]!='.')continue;
			G[nextH*W+nextW].emplace_back(i*W+j,1);
			G[i*W+j].emplace_back(nextH*W+nextW,1);
		}
	}
	vector<vector<ll>>v(H*W);
	for(int i=0;i<H*W;i++){
		v[i]=dijkstra(G,i);
	}
	ll N=H*W;
	ll ans=0;
	rep(i,N)rep(j,N){
		if(v[i][j]>pow(10,9))continue;
		ans=max(ans,v[i][j]);
	}
	cout<<ans<<endl;
}


Measured memory_bytes: 1536