#include <bits/stdc++.h>
#define all(v) v.begin(), v.end()
#define rall(v) v.rbegin(), v.rend()
#define rep(i,n) for (int i=0;i<(int)(n);i++)
#define codefor int test;scanf("%d",&test);while(test--)
#define yn(ans) if(ans)printf("Yes\n");else printf("No\n")
#define YN(ans) if(ans)printf("YES\n");else printf("NO\n")
#define vector2d(type,name,h,...) vector<vector<type>>name(h,vector<type>(__VA_ARGS__))
#define umap unordered_map
#define uset unordered_set
using namespace std;
using ll = long long;
const int MOD=1000000007;
//入力系
void scan(int& a){scanf("%d",&a);}
void scan(long long& a){scanf("%lld",&a);}
template<class T> void scan(T& a){cin>>a;}
template<class T> void scan(vector<T>& vec){for(auto&& it:vec)scan(it);}
void in(){}
template <class Head, class... Tail> void in(Head& head, Tail&... tail){scan(head);in(tail...);}
//出力系
void print(const int& a){printf("%d",a);}
void print(const long long& a){printf("%lld",a);}
void print(const double& a){printf("%.15lf",a);}
template<class T> void print(const T& a){cout<<a;}
template<class T> void print(const vector<T>& vec){if(vec.empty())return;print(vec[0]);for(auto it=vec.begin();++it!= vec.end();){putchar(' ');print(*it);}}
void out(){putchar('\n');}
template<class T> void out(const T& t){print(t);putchar('\n');}
template <class Head, class... Tail> void out(const Head& head,const Tail&... tail){print(head);putchar(' ');out(tail...);}
//デバッグ系
template<class T> void dprint(const T& a){cerr<<a;}
template<class T> void dprint(const vector<T>& vec){if(vec.empty())return;cerr<<vec[0];for(auto it=vec.begin();++it!= vec.end();){cerr<<" "<<*it;}}
void debug(){cerr<<endl;}
template<class T> void debug(const T& t){dprint(t);cerr<<endl;}
template <class Head, class... Tail> void debug(const Head& head, const Tail&... tail){dprint(head);cerr<<" ";debug(tail...);}
ll intpow(ll a, ll b){ ll ans = 1; while(b){ if(b & 1) ans *= a; a *= a; b /= 2; } return ans; }
ll modpow(ll a, ll b, ll p){ ll ans = 1; while(b){ if(b & 1) (ans *= a) %= p; (a *= a) %= p; b /= 2; } return ans; }

int main(){
    int h,w,maxv1=0,maxv2=0,x,y;
    in(h,w);
    vector2d(char,maze,h,w);
    in(maze);
    rep(sy,h){
        rep(sx,w){
            maxv2=0;
            //debug("enter");
            if(maze[sy][sx]=='.'){
                queue<pair<int,int>> next;
                vector2d(int,solu,h,w,-1);
                solu[sy][sx]=0;
                next.push(make_pair(sy,sx));
                while(!next.empty()){
                    //debug("enter");
                    y=next.front().first;
                    x=next.front().second;
                    maxv2=max(maxv2,solu[y][x]);
                    next.pop();
                    if(x-1>=0&&solu[y][x-1]==-1&&maze[y][x-1]=='.'){
                        next.push(make_pair(y,x-1));
                        solu[y][x-1]=solu[y][x]+1;
                    }
                    if(y-1>=0&&solu[y-1][x]==-1&&maze[y-1][x]=='.'){
                        next.push(make_pair(y-1,x));
                        solu[y-1][x]=solu[y][x]+1;
                    }
                    if(x+1<w&&solu[y][x+1]==-1&&maze[y][x+1]=='.'){
                        next.push(make_pair(y,x+1));
                        solu[y][x+1]=solu[y][x]+1;
                    }
                    if(y+1<h&&solu[y+1][x]==-1&&maze[y+1][x]=='.'){
                        next.push(make_pair(y+1,x));
                        solu[y+1][x]=solu[y][x]+1;
                    }
                }
                maxv1=max(maxv1,maxv2);
            }
        }
    }
    out(maxv1);
}


Measured memory_bytes: 3776