#include<bits/stdc++.h>
using namespace std;
typedef long long ll;
#define F first
#define S second
#define pii pair<int, int>
#define pli pair<ll, int>
#define pil pair<int, ll>
#define pll pair<ll, ll>
#define eb emplace_back
#define all(v) v.begin(), v.end()
#define rep(i, n) for (int i = 0; i < (n); ++i)
#define rep3(i, l, n) for (int i = l; i < (n); ++i)
#define sz(v) (int)v.size()
#define endl '\n'
const int inf = 1000000007;
const ll INF = 1e18;
// int mod = 998244353;
int mod = 1000000007;
const double eps = 1e-8;
#define abs(x) (x >= 0 ? x : -(x))
#define lb(v, x) (int)(lower_bound(all(v), x) - v.begin())
#define ub(v, x) (int)(upper_bound(all(v), x) - v.begin())
template<typename T1, typename T2> inline bool chmin(T1 &a, T2 b) { if (a > b) { a = b; return 1; } return 0; }
template<typename T1, typename T2> inline bool chmax(T1 &a, T2 b) { if (a < b) { a = b; return 1; } return 0; }
ll gcd(ll a, ll b) { if (b == 0) return a; return gcd(b, a % b); }
ll lcm(ll a, ll b) { return a / gcd(a, b) * b; }
template<typename T, typename U> T pow_(T a, U b) { return b ? pow_(a * a, b / 2) * (b % 2 ? a : 1) : 1; }
ll modpow(ll a, ll b, ll _mod) { return b ? modpow(a * a % _mod, b / 2, _mod) * (b % 2 ? a : 1) % _mod : 1; }
template<class T, class U> ostream& operator << (ostream& os, const pair<T, U>& p) { os << p.F << " " << p.S; return os; }
template<class T> ostream& operator << (ostream& os, const vector<T>& vec) { rep(i, sz(vec)) { if (i) os << " "; os << vec[i]; } return os; }
template<class T> ostream& operator << (ostream& os, const set<T>& s) { for (auto si : s) os << si << " "; return os; }
template<typename T> inline istream& operator >> (istream& is, vector<T>& v) { rep(j, sz(v)) is >> v[j]; return is; }
template<class T, class T2> inline void add(T &a, T2 b) { (a += (b % mod + mod)) %= mod; }
template<class T> void operator += (vector<T>& v, vector<T> v2) { rep(i, sz(v2)) v.eb(v2[i]); }

void solve();

int main() {
    ios::sync_with_stdio(false);
    cin.tie(0);
    cout.tie(0);
    cout << fixed << setprecision(12);
    int T;
    T = 1;
    while (T--) solve();
}

void solve() {
    int h, w;
    cin >> h >> w;
    vector<vector<int> > a(h, vector<int>(w));
    rep(i, h) rep(j, w) { char c; cin >> c; a[i][j] = (c == '.' ? 0 : 1); }
    
    int dy[] = {-1, 0, 1, 0};
    int dx[] = {0, 1, 0, -1};
    int ans = 0;
    rep(sy, h) rep(sx, w) {
        if (a[sy][sx] == 1) continue;
        rep(gy, h) rep(gx, w) {
            if (a[gy][gx] == 1) continue;
            queue<pii> q;
            vector<vector<int> > d(h, vector<int>(w, inf));
            q.emplace(sy, sx);
            d[sy][sx] = 0;
            while (!q.empty()) {
                pii now = q.front();
                int y = now.F, x = now.S;
                q.pop();
                rep(k, 4) {
                    int ny = y + dy[k];
                    int nx = x + dx[k];
                    if (ny < 0 || ny >= h || nx < 0 || nx >= w) continue;
                    if (a[ny][nx] == 1) continue;
                    if (chmin(d[ny][nx], d[y][x] + 1)) {
                        q.emplace(ny, nx);
                    }
                }
            }
            chmax(ans, d[gy][gx]);
        }
    }
    cout << ans << endl;
}


Measured memory_bytes: 3624