#include <bits/stdc++.h>
#define rep(i,n) for(int i=0;i<(n);i++)
#define all(v) v.begin(),v.end()
using namespace std;
typedef long long ll;
typedef pair<int,int> P;
const int INF=1e9;
const int di[] = {-1,0,1,0};
const int dj[] = {0,-1,0,1};

int main() {
  int h,w;
  cin>>h>>w;
  vector<string> s(h);
  rep(i,h) cin>>s[i];
  int ans=0;
  rep(si,h)rep(sj,w){
    if(s[si][sj]=='#') continue;
    vector<vector<int>> dist(h,vector<int>(w,INF));
    queue<P> q;
    auto update=[&](int i,int j,int x){
      dist[i][j]=x;
      q.push(P(i,j));// .emplace(i,j)
    };
    update(si,sj,0);
    while(!q.empty()){
      int i=q.front().first;
      int j=q.front().second; q.pop();
      rep(dir,4){
        int ni=i+di[dir], nj=j+dj[dir];
        if(ni<0||ni>=h||nj<0||nj>=w) continue;
        if(s[ni][nj]=='#') continue;
        if(dist[ni][nj]!=INF) continue;
        update(ni,nj,dist[i][j]+1);
      }
    }
    rep(i,h)rep(j,w){
      if(dist[i][j]==INF) continue;
      ans=max(ans,dist[i][j]);
    }
  }
  cout<<ans<<endl;
}