#include <bits/stdc++.h>
#define fi first
#define se second
#define rep(i, n) for (int i = 0; i < (n); ++i)
#define rrep(i, n) for (int i = 1; i <= (n); ++i)
#define drep(i, n) for (int i = (n)-1; i >= 0; --i)
#define srep(i, s, t) for (int i = s; i < t; ++i)
#define rng(a) a.begin(), a.end()
#define maxs(x, y) (x = max(x, y))
#define mins(x, y) (x = min(x, y))
#define pb push_back
#define sz(x) (int)(x).size()
#define PQ(T) priority_queue<T, v(T), greater<T>>
#define bn(x) ((1 << x) - 1)
#define v(T) vector<T>
#define vv(T) v(v(T))
using namespace std;
typedef long long int ll;
typedef unsigned uint;
typedef unsigned long long ull;
typedef pair<int, int> P;
typedef vector<int> vi;
typedef vector<vi> vvi;
typedef vector<ll> vl;
typedef vector<P> vp;
const double eps = 1e-10;
const ll LINF = 1001002003004005006ll;
const int INF = 1001001001;

const int mod = 1000000007;

struct mint
{
    ll x;
    mint(ll x = 0) : x((x % mod + mod) % mod){};
    mint operator-() const
    {
        return mint(-x);
    }
    mint &operator+=(const mint a)
    {
        if ((x += a.x) >= mod)
            x -= mod;
        return *this;
    }
    mint &operator-=(const mint a)
    {
        if ((x += mod - a.x) >= mod)
            x -= mod;
        return *this;
    }
    mint &operator*=(const mint a)
    {
        (x *= a.x) %= mod;
        return *this;
    }
    mint operator+(const mint a) const
    {
        mint res(*this);
        return res += a;
    }
    mint operator-(const mint a) const
    {
        mint res(*this);
        return res -= a;
    }
    mint operator*(const mint a) const
    {
        mint res(*this);
        return res *= a;
    }
    mint pow(ll t) const
    {
        if (!t)
            return 1;
        mint a = pow(t >> 1);
        a *= a;
        if (1 & t)
            a *= *this;
        return a;
    }

    mint inv() const
    {
        return pow(mod - 2);
    }
    mint &operator/=(const mint a)
    {
        return (*this) *= a.inv();
    }
    mint operator/(const mint a) const
    {
        mint res(*this);
        return res /= a;
    }
};

struct combination
{
    vector<mint> fact, ifact;
    combination(int n) : fact(n + 1), ifact(n + 1)
    {
        assert(n < mod);
        fact[0] = 1;
        for (int i = 1; i <= n; i++)
            fact[i] = fact[i - 1] * i;
        ifact[n] = fact[n].inv();
        for (int i = n; i >= 1; i--)
            ifact[i - 1] = ifact[i] * i;
    }
    mint operator()(int n, int k)
    {
        if (k < 0 || k > n)
            return 0;
        return fact[n] * ifact[k] * ifact[n - k];
    }
};

const int di[] = {0, -1, 0, 1};
const int dj[] = {-1, 0, 1, 0};

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;

        queue<P> q;
        vvi dist(h, vi(w, INF));
        auto update = [&](int i, int j, int x) {
            if (dist[i][j] != INF)
                return;
            dist[i][j] = x;
            q.push(P(i, j));
        };

        update(si, sj, 0);

        while(!q.empty()){
            int i = q.front().fi;
            int j = q.front().se;
            q.pop();

            rep(dir, 4){
                int ni = i + di[dir];
                int nj = j + dj[dir];
                if(ni<0||ni>=h||nj<0||nj>=w) continue;
                if(s[ni][nj]=='#') continue;
                update(ni, nj, dist[i][j]+1);
            }
        }

        rep(i, h){
            rep(j, w){
                //cout << dist[i][j] << " ";
                if(dist[i][j]==INF) continue;
                ans = max(ans, dist[i][j]);
            }
            //cout << endl;
        }
    }

    cout << ans << endl;
    return 0;
}

Measured memory_bytes: 256