#include <bits/stdc++.h>
#define rep(i, start, end) for (long long i = start; i < end; ++i)
#define repreverse(i, start, end) for (long long i = start; i >= end; --i)
#define all(x) (x).begin(), (x).end()
#define rall(x) (x).rbegin(), (x).rend()
#define len(x) ((long long)(x).size())
#define lcm(a, b) ((a) / __gcd((a), (b)) * (b))
using namespace std;
using ll = long long;
using ld = long double;
using vll = vector<ll>;
using vllvll = vector<vll>;
using vc = vector<char>;
using pll = pair<ll, ll>;
template<class T> void print1d(T x,ll n=-1){if(n==-1)n=x.size();rep(i,0,n){cout<<x[i]<<' ';}cout<<'\n';}
template<class T> void print2d(T x,ll r=-1,ll c=-1){if(r==-1)r=x.size();if(c==-1)c=x[0].size();rep(i,0,r)print1d(x[i],c);}
template<class T, class U> bool haskey(T mp, U key) { return mp.find(key) != mp.end(); }
template<class T, class U> bool isin(T el, U container) { return find(all(container), el) != container.end(); }
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<class T> bool even(T n) { return ! (n & 1); }
template<class T> bool odd(T n) { return n & 1; }
template<class T> ll rup(T a, T b) { return a % b ? a / b + 1 : a / b; }
template<class T> ld deg2rad(T deg) { return M_PI * deg / 180.0; }
template<class T> ld rad2deg(T rad) { return 180.0 * rad / M_PI; }
const long double pi = M_PI;
const long long big = 1LL << 50;
const long long inf = 1LL << 60;
const long long mod = 1e9 + 7;
ll intpow(ll a, ll n, ll _mod=numeric_limits<ll>::max()) { ll p=1; while (n) { if (n&1) p=p*a%_mod; a=a*a%_mod; n>>=1; } return p; } 
ll modc(ll a, char op, ll b, ll _mod=mod)
{
        a %= _mod; b %= _mod; ll res = 1;
        switch (op) {
        case '+': res = (a + b) % _mod; break;
        case '-': res = (a - b) % _mod; break;
        case '*': res = a * b % _mod; break;
        case '/': res = modc(a, '*', modc(b, '^', _mod-2, _mod), _mod); break;
        case '^': res = intpow(a, b, _mod); break;
        case 'P': rep(i, a-b+1, a+1) res = modc(res, '*', i, _mod); break;
        case 'C': res = modc(modc(a, 'P', b, _mod), '/', modc(b, 'P', b, _mod)); break;
        } 
        if (res < 0) { res += _mod; } return res;
}



int main()
{
        ll H, W;
        cin >> H >> W;
        vector<vc> grid(H+2, vc(W+2, '#'));

        rep(i, 1, H+1) rep(j, 1, W+1) cin >> grid[i][j];

        ll ans = -inf;

        vll dy_ = {0, 1, 0, -1};
        vll dx_ = {-1, 0, 1, 0};

        rep(sy, 1, H+1) {
                rep(sx, 1, W+1) {

                        if (grid[sy][sx] == '#') continue;
                        //cout << sy << ' '  << sx << ' ' << gy << ' ' << gx << endl;
                        auto s = pll{sy, sx};
                        queue<pll> q;
                        q.push(s);

                        vllvll dist(H+2, vll(W+2, inf));
                        dist[sy][sx] = 0;

                        while (not q.empty()) {
                                auto v = q.front(); q.pop();
                                ll y = v.first;
                                ll x = v.second;

                                rep(d, 0, 4) {
                                        ll dy = dy_[d];
                                        ll dx = dx_[d];
                                        if (dist[y+dy][x+dx] != inf) continue;
                                        if (grid[y+dy][x+dx] == '#') continue;

                                        dist[y+dy][x+dx] = dist[y][x] + 1;
                                        q.push(pll{y+dy, x+dx});
                                }
                                
                        }

                        ll mx = -inf;
                        rep(i, 0, H+2) rep(j, 0, W+2) if (dist[i][j] != inf) chmax(mx, dist[i][j]);
                        chmax(ans, mx);

                }
        }

        cout << ans << endl;
}


Measured memory_bytes: 3652