#include <bits/stdc++.h>
using namespace std;

#define int long long
typedef pair<int, int> P;
#define mod 1000000007
#define INF (1LL<<60)
 
#define rep(i,n) for(int i=0, i##_len=(n); i<i##_len; ++i)
#define repi(itr, ds) for (auto itr = ds.begin(); itr != ds.end(); itr++)
#define YES puts("YES")
#define Yes puts("Yes")
#define NO  puts("NO")
#define No  puts("No")
int gcd(int a, int b) { return b ? gcd(b, a % b) : a; }
 
const int dx[4] = { 1, 0, -1, 0 };
const int dy[4] = { 0, 1, 0, -1 };
 
 
#if 1
//-------------
// DUMPマクロ
// https://www.creativ.xyz/dump-cpp-652/
 
// vector
template <typename T>
istream& operator>>(istream& is, vector<T>& vec) {
    for (T& x : vec) is >> x;
    return is;
}
// pair
template <typename T, typename U>
ostream& operator<<(ostream& os, pair<T, U>& pair_var) {
    os << "(" << pair_var.first << ", " << pair_var.second << ")";
    return os;
}
// vector
template <typename T>
ostream& operator<<(ostream& os, const vector<T>& vec) {
    os << "{";
    for (int i = 0; i < vec.size(); i++) {
        os << vec[i] << (i + 1 == vec.size() ? "" : ", ");
    }
    os << "}";
    return os;
}
// map
template <typename T, typename U>
ostream& operator<<(ostream& os, map<T, U>& map_var) {
    os << "{";
    repi(itr, map_var) {
        os << *itr;
        itr++;
        if (itr != map_var.end()) os << ", ";
        itr--;
    }
    os << "}";
    return os;
}
// set
template <typename T>
ostream& operator<<(ostream& os, set<T>& set_var) {
    os << "{";
    repi(itr, set_var) {
        os << *itr;
        itr++;
        if (itr != set_var.end()) os << ", ";
        itr--;
    }
    os << "}";
    return os;
}
 
#define DUMPOUT cerr
 
void dump_func() {
    DUMPOUT << endl;
}
template <class Head, class... Tail>
void dump_func(Head&& head, Tail&& ... tail) {
    DUMPOUT << head;
    if (sizeof...(Tail) > 0) {
        DUMPOUT << ", ";
    }
    dump_func(std::move(tail)...);
}
#ifdef DEBUG
#define DEB
#define dump(...)                                                              \
    DUMPOUT << "  " << string(#__VA_ARGS__) << ": "                            \
            << "[" << to_string(__LINE__) << ":" << __FUNCTION__ << "]"        \
            << endl                                                            \
            << "    ",                                                         \
        dump_func(__VA_ARGS__)
#else
#define DEB if (false)
#define dump(...)
#endif
#endif




int H, W;
int sh, sw;
char s[22][22];
int d[22][22];

void bfs(){
    queue<P> que;

    for (int h = 1; h <=H; h++)
        for (int w = 1; w <= W; w++) d[h][w] = INF;
    
    que.push(P(sh, sw));
    d[sh][sw] = 0;
    
    while (que.size()>0){
        P p = que.front();
        que.pop();
        
        for (int i = 0; i < 4; i++){
            int ny = p.first + dy[i], nx = p.second + dx[i];
            if (s[ny][nx] == '.' && d[ny][nx] == INF){
                que.push(P(ny, nx));
                d[ny][nx] = d[p.first][p.second] + 1;
            }
        }
    }
}

signed main(){
    cin >> H >> W;
    
    for (int h = 1; h <= H; h++)
        for (int w = 1; w <= W; w++)
            cin >> s[h][w];

    int ans = 0;
    for( sh = 1; sh <= H; sh++){
        for( sw = 1; sw<=W; sw++){
            
            if(s[sh][sw]=='#') continue;
            
            bfs();
            
            for(int h=1; h<=H; h++)
                for(int w=1; w<=W; w++)
                    if(d[h][w]!=INF) ans = max(ans, d[h][w]);

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


Measured memory_bytes: 256