#include <bits/stdc++.h>

using namespace std;

#define rep(i, n) for(int i = 0; i < (n); ++i)
#define per(i, n) for(int i = (n)-1; i >= 0; --i)
#define eee(a) (cerr << (#a) << ": " << (a) << endl);
#define ppp(a) (cout << (a) << endl); 
#define cop(a) for(auto x:(a)){cout << x << endl;}

using ll = long long;

constexpr int dx[4] = {1, 0, -1, 0};
constexpr int dy[4] = {0, 1, 0, -1};

constexpr char sp = ' ';
constexpr ll MOD = 1e9+7;
constexpr ll Mod = 998244353;
constexpr int INF = 1e9;
constexpr ll LINF = 1e18;
constexpr double phi = (1+sqrt(5))/2;
constexpr double pi = 3.14159265358979323846264338327950288;
inline ll gcd(ll a, ll b){return b?gcd(b,a%b):a;}
inline ll lcm(ll a, ll b){return a/gcd(a,b)*b;}
inline ll inv(ll n, ll m=MOD){ll a=n,b=m,x=1,y=0;while(b){ll t=a/b;a-=t*b;swap(a,b);x-=t*y;swap(x,y);}return (a%m+m)%m;}
inline void Yes(){puts("Yes");}
inline void No(){puts("No");}
inline void Judge(bool b){b?Yes():No();}
inline void YES(){puts("YES");}
inline void NO(){puts("NO");}
inline void JUDGE(bool b){b?YES():NO();}
inline ll powmod(ll b, ll e, ll m=MOD){ll r=1;while(e>0){if(e&1)r=(r%m)*(b%m)%m;b=(b%m)*(b%m)%m;e>>=1;}return r;}
inline double distance(ll x1, ll y1, ll x2, ll y2){return sqrt((x1-x2)*(x1-x2)+(y1-y2)*(y1-y2));}
inline double d2r(double d){return d*pi/180;}
inline double r2d(double r){return r*180/pi;}

template<class T>bool chmax(T &a, const T &b) { if (a<b) { a=b; return 1; } return 0; }
template<class T>bool chmin(T &a, const T &b) { if (b<a) { a=b; return 1; } return 0; }
template<typename T1, typename T2> ostream& operator<<(ostream& s, const pair<T1, T2>& p) {return s << "(" << p.first << ", " << p.second << ")";}
template<typename T> ostream& operator<<(ostream& s, const vector<T>& v) {
    s<<'[';for(size_t i=0;i<v.size();++i){s << v[i];if(i < v.size()-1) s << ", ";}s<<']';return s;
}
struct __{ __(){ cin.tie(0); ios::sync_with_stdio(0); cout<<fixed<<setprecision(15); } };

vector<string> s;
vector<vector<int>> step;
vector<vector<int>> step2;
int main(){

    int h, w;
    scanf("%d%d", &h, &w);
    s.resize(h);
    step.resize(h, vector<int>(w, -1));
    step2.resize(h, vector<int>(w, -1));
    int stx = -1, sty = -1;
    rep(i, h){
        cin >> s[i];
        rep(j, w){
            if(stx == -1 and sty == -1 and s[i][j] == '.'){
                stx = j; sty = i;
            }
        }
    }

    queue<pair<int, int>> q;
    q.emplace(stx, sty);
    step[sty][stx] = 0;
    while(!q.empty()){
        pair<int, int> xy = q.front(); q.pop();
        int x = xy.first, y = xy.second;
        rep(i, 4){
            int X = x+dx[i], Y = y+dy[i];
            if(X < 0 or X >= w or Y < 0 or Y >= h) continue;
            if(step[Y][X] >= 0) continue;
            if(s[Y][X] == '#') continue;
            step[Y][X] = step[y][x] + 1;
            q.emplace(X, Y);
        }
    }

    int tmpmax = 0;
    rep(i, h){
        rep(j, w){
            if(step[i][j] > tmpmax){
                stx = j; sty = i;
                tmpmax = step[i][j];
            }
        }
    }


    queue<pair<int, int>> q2;
    q2.emplace(stx, sty);
    step2[sty][stx] = 0;
    while(!q2.empty()){
        pair<int, int> xy = q2.front(); q2.pop();
        int x = xy.first, y = xy.second;
        rep(i, 4){
            int X = x+dx[i], Y = y+dy[i];
            if(X < 0 or X >= w or Y < 0 or Y >= h) continue;
            if(step2[Y][X] >= 0) continue;
            if(s[Y][X] == '#') continue;
            step2[Y][X] = step2[y][x] + 1;
            q2.emplace(X, Y);
        }
    }
    int ans = 0;
    rep(i, h){
        rep(j, w){
            chmax(ans, step2[i][j]);
        }
    }

    ppp(ans);




    return 0;
}
