#define _USE_MATH_DEFINES
#include <algorithm>
#include <bitset>
#include <cassert>
#include <cctype>
#include <clocale>
#include <cmath>
#include <cstdlib>
#include <ctime>
#include <deque>
#include <fstream>
#include <functional>
#include <iomanip>
#include <iostream>
#include <iterator>
#include <list>
#include <map>
#include <queue>
#include <regex>
#include <set>
#include <sstream>
#include <stack>
#include <string>
#include <tuple>
#include <unordered_map>
#include <unordered_set>
#include <utility>
#include <vector>

using namespace std;

#define IOS ios::sync_with_stdio(false); cin.tie(0);
#define FOR(i, s, n) for(int i = (s), i##_len=(n); i < i##_len; ++i)
#define FORS(i, s, n) for(int i = (s), i##_len=(n); i <= i##_len; ++i)
#define VFOR(i, s, n) for(int i = (s); i < (n); ++i)
#define VFORS(i, s, n) for(int i = (s); i <= (n); ++i)
#define REP(i, n) FOR(i, 0, n)
#define REPS(i, n) FORS(i, 0, n)
#define VREP(i, n) VFOR(i, 0, n)
#define VREPS(i, n) VFORS(i, 0, n)
#define RFOR(i, s, n) for(int i = (s), i##_len=(n); i >= i##_len; --i)
#define RFORS(i, s, n) for(int i = (s), i##_len=(n); i > i##_len; --i)
#define RREP(i, n) RFOR(i, n, 0)
#define RREPS(i, n) RFORS(i, n, 0)
#define ALL(v) (v).begin(), (v).end()
#define SORT(v) sort(ALL(v))
#define RSORT(v) sort(ALL(v), greater<decltype(v[0])>())
#define SZ(x) ((int)(x).size())
#define PB push_back
#define EB emplace_back
#define MP make_pair
#define MT make_tuple
#define BIT(n) (1LL<<(n))
#define UNIQUE(v) v.erase(unique(ALL(v)), v.end())

using ll = long long;
using ui = unsigned int;
using ull = unsigned long long;
using Pi_i = pair<int, int>;
using VB = vector<bool>;
using VC = vector<char>;
using VD = vector<double>;
using VI = vector<int>;
using VLL = vector<ll>;
using VS = vector<string>;
using VSH = vector<short>;
using VULL = vector<ull>;

const int MOD = 1000000007;
const int INF = 1000000000;
const int NIL = -1;
const double EPS = 1E-10;

template<class T, class S>
bool chmax(T &a, const S &b){
    if(a < b){
        a = b; return true;
    }
    return false;
}
template<class T, class S>
bool chmin(T &a, const S &b){
    if(b < a){
        a = b; return true;
    }
    return false;
}


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

int main(){
    int H, W; cin >> H >> W;
    VS S(H);
    REP(i, H) cin >> S[i];
    int ans = 0;
    REP(i, H){
        REP(j, W){
            if(S[i][j] == '#') continue;
            REP(gi, H){
                REP(gj, W){
                    if(S[gi][gj] == '#') continue;
                    if(i == gi && j == gj) continue;
                    vector<VI> d(H, VI(W, INF));
                    queue<Pi_i> que;
                    que.push(Pi_i(i, j));
                    d[i][j] = 0;
                    while(!que.empty()){
                        Pi_i p = que.front(); que.pop();
                        REP(k, 4){
                            int ni = p.first + dx[k];
                            int nj = p.second + dy[k];
                            if(0<=ni && ni<H && 0<=nj && nj<W
                               && S[ni][nj]=='.' && d[ni][nj] == INF){
                                d[ni][nj] = d[p.first][p.second] + 1;
                                if(ni == gi && nj == gj)break;
                                que.push(Pi_i(ni, nj));
                            }
                        }
                    }
                    chmax(ans, d[gi][gj]);
                }
            }
        }
    }
    cout << ans << endl;
    return 0;
}


Measured memory_bytes: 256