#define _GLIBCXX_DEBUG
// header {{{
#include <bits/stdc++.h>
using namespace std;
#define CPP_STR(x) CPP_STR_I(x)
#define CPP_CAT(x,y) CPP_CAT_I(x,y)
#define CPP_STR_I(args...) #args
#define CPP_CAT_I(x,y) x ## y
#define ASSERT(expr...) assert((expr))
using i8  = int8_t;
using u8  = uint8_t;
using i16 = int16_t;
using u16 = uint16_t;
using i32 = int32_t;
using u32 = uint32_t;
using i64 = int64_t;
using u64 = uint64_t;
using f32 = float;
using f64 = double;
using f80 = __float80;
// }}}
constexpr i64 INF = 1'010'000'000'000'000'017LL;
constexpr i64 MOD = 1'000'000'007LL;
constexpr f64 EPS = 1e-12;
constexpr f64 PI = 3.14159265358979323846;
// util {{{
#define FOR(i, start, end) for(i64 i = (start), CPP_CAT(i,xxxx_end)=(end); i < CPP_CAT(i,xxxx_end); ++i)
#define REP(i, n) FOR(i, 0, n)
#define ALL(f,c,...) (([&](decltype((c)) cccc) { return (f)(std::begin(cccc), std::end(cccc), ## __VA_ARGS__); })(c))

template<typename C>
i64 SIZE(const C& c) { return static_cast<i64>(c.size()); }
template<typename T, size_t N>
i64 SIZE(const T (&)[N]) { return static_cast<i64>(N); }
template<typename T, typename U, typename Comp=less<>>
bool chmax(T& xmax, const U& x, Comp comp={}) {
    if(comp(xmax, x)) {
        xmax = x;
        return true;
    }
    return false;
}

template<typename T, typename U, typename Comp=less<>>
bool chmin(T& xmin, const U& x, Comp comp={}) {
    if(comp(x, xmin)) {
        xmin = x;
        return true;
    }
    return false;
}
// }}}

// init {{{
struct ProconInit {
    static constexpr int IOS_PREC = 15;
    static constexpr bool AUTOFLUSH = false;

    ProconInit() {
        cin.tie(nullptr);
        ios::sync_with_stdio(false);
        cout << fixed << setprecision(IOS_PREC);
        if(AUTOFLUSH)
            cout << unitbuf;
    }
} PROCON_INIT;
// }}}

//--------------------------------------------------------------------

int h,w;
signed main() {
    cin>>h>>w;
    vector< vector<char> > s(h, vector<char> (w));

    vector< vector<int> > l(h, vector<int> (w, 0) );
    

    int n = h*w;

    REP(i,h){
        REP(j,w){
            cin>>s[i][j];
        }
    }
    
    typedef pair<int,int> P;

    i64 mx = 0;

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

    i64 d[h][w];

    REP(ist,h){
        REP(jst,w){
            if( s[ist][jst] == '#') continue;

            queue<P> que;
            REP(i,h){
                REP(j,w){
                    d[i][j] = INF;
                }
            }
            
            que.push(P(ist,jst));
            d[ist][jst] = 0;

            while(que.size()){
                P p = que.front(); que.pop();

                REP(k,4){
                    int nx = p.first + dx[k], ny = p.second + dy[k];
                    if(0<= nx && nx < h && 0 <= ny && ny < w && s[nx][ny] != '#' && d[nx][ny] == INF){
                        que.push(P(nx,ny));
                        d[nx][ny] = d[p.first][p.second]+1;
                    }
                }

            }
            
            
            REP(i,h){
                REP(j,w){
                    if(s[i][j] == '#') continue;
                    mx = max(mx,d[i][j]);
                }
            }
            
            
        }
    }
    cout<<mx<<endl;

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

Measured memory_bytes: 256