#pragma region header
#include <bits/stdc++.h>
using namespace std;
#define int long long
#define rep(i, n) for (int i = 0; i < (int)(n); i++)
#define rep1(i, n) for (int i = 1; i <= (int)(n); i++)
#define rev(i, n) for(int i = (int)(n - 1); i >= 0; i--)
#define rev1(i, n) for(int i = (int)(n); i > 0; i--)
#define pb push_back
#define all(v) (v).begin(), (v).end()
#define resort(v) sort((v).rbegin(), (v).rend())
#define vi vector<int>
#define vvi vector<vector<int>>
#define vc vector<char>
#define vvc vector<vector<char>>
#define vb vector<bool>
#define vvb vector<vector<bool>>
using ll = long long;
using P = pair<int, int>;
/* ----------------よく使う数字や配列----------------- */
int dx[] = {1,0,-1,0};
int dy[] = {0,1,0,-1};
constexpr ll mod = 1e9+7;
constexpr ll inf = INT32_MAX/2;
constexpr ll INF = LLONG_MAX/2;
constexpr long double eps = DBL_EPSILON;
constexpr long double pi = 3.141592653589793238462643383279;
/* ----------------------end----------------------- */

/* --------------------テンプレート------------------ */
template<class T> inline bool chmin(T& a, T b) {
    if (a > b) {
        a = b;
        return true;
    }
    return false;
}
template<class T> inline bool chmax(T& a, T b) {
    if (a < b) {
        a = b;
        return true;
    }
    return false;
}
/* ----------------------end----------------------- */

/* --------------------ライブラリ-------------------- */
ll fact(int i) {       //階乗
    if (i == 0) return 1;
    return (fact(i - 1)) * i % mod;
}
ll gcd(ll a, ll b) {        //最大公約数
    if(b == 0) return a;
    return gcd(b, a % b); 
}
ll lcm(ll a, ll b) {      //最小公倍数
    return a * b / gcd(a, b);
}
int keta(ll n) {        //桁数を求める
    if(n == 0) return 1;
    int count = 0;
    while(n != 0) {
        n /= 10;
        count++;
    }
    return count;
}
ll ketasum(ll n) {    //各桁の和
    ll sum = 0;
    while(n != 0) {
        sum += n % 10;
        n /= 10;
    }
    return sum;
}
/* ----------------------end----------------------- */
#pragma endregion
signed main() { 
    int h,w;cin >> h >> w;
    vvb Bo(h,vb(w,false));
    vvi v(h,vi(w,inf));
    rep(i, h) {
        rep(j, w) {
            char c;cin >> c;
            if(c=='#') {
                Bo[i][j]=true;
                v[i][j]=-1;
            }
        }
    }
    int Max=-1;
    rep(i, h) {
        rep(j, w) {
            if(Bo[i][j]) continue;
            vvb bo=Bo;
            vvi ans = v;
            int ma = -1;
            queue<P> que;
            que.push({i,j});
            ans[i][j]=0;
            bo[i][j]=true;
            while(!que.empty()) {
                P p = que.front();
                que.pop();
                rep(k, 4) {
                    int x = p.first+dx[k],y=p.second+dy[k];
                    if(x<0||x>=h||y<0||y>=w||bo[x][y]) continue;
                    ans[x][y]=ans[p.first][p.second]+1;
                    chmax(ma,ans[x][y]);
                    que.push({x,y});
                    bo[x][y]=true;
                    // cout << ma << ' ';
                    // cout << k << endl;
                    // cout << "(x,y)=("<< x << "," << y << ")\n";
                }
            }
            /*if(*/chmax(Max,ma)/*)*//*cout <<Max << ' ' <<  "(i,j)=("<< i << "," << j << ")\n"*/;;
        }
    }
    cout << Max << endl;
    return 0;
}

Measured memory_bytes: 3616