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

#define fastio ios_base::sync_with_stdio(false);cin.tie(NULL);cout.tie(NULL); 

#define ll long long
#define ff first
#define ss second
#define pb push_back
#define pf push_front
#define mp make_pair
#define pu push
#define pp pop_back
#define in insert
#define ld long double
#define forn(low,high,i) for(i = low;i < high;i++)
#define forrev(high,low,i) for(i = high; i >= low;i--)
#define all(v) v.begin(),v.end()
#define sz(v) (int)v.size()
#define line cout << __LINE__;
#define prv(a) for(auto x : a) cout << x << ' ';cout << '\n';
#define prvp(a) for(auto x : a) cout << "{" << x.ff << ',' << x.ss << "}";cout << '\n';
#define decimal_digits cout << fixed << setprecision(15);
string to_string(string s) { return '"' + s + '"';}
string to_string(char s) { return string(1, s);}
string to_string(const char* s) { return to_string((string) s);}
string to_string(bool b) { return (b ? "true" : "false");}
template <typename A> string to_string(A);
template <typename A, typename B>string to_string(pair<A, B> p) {return "(" + to_string(p.first) + ", " + to_string(p.second) + ")";}
template <typename A> string to_string(A v) {bool f = 1; string r = "{"; for (const auto &x : v) {if (!f)r += ", "; f = 0; r += to_string(x);} return r + "}";}
void debug_out() { cerr << endl; }
template <typename Head, typename... Tail> void debug_out(Head H, Tail... T) {cerr << " " << to_string(H); debug_out(T...);}
#define pr(...) cerr << "[" << #__VA_ARGS__ << "]:", debug_out(__VA_ARGS__)

typedef unordered_map<int,int> umi;
typedef unordered_map<ll,ll> uml;
typedef vector<int> vi;
typedef vector<ll> vl;
typedef vector<vi> vvi;
typedef vector<vl> vvl;
typedef pair<int,int> pii;
typedef pair<ll,ll> pll;
typedef vector<pii> vpii;
typedef vector<pll> vpll;

mt19937_64 rng(chrono::steady_clock::now().time_since_epoch().count());

const int inf = 1e9;
const ll INF = 1e18;
const int mod = 1e9 + 7;
const int bit32 = log2(inf) + 3;
const int bit64 = log2(INF) + 3;

inline int add(int a, int b){a += b; if(a >= mod) a -= mod; return a;}
inline int sub(int a, int b){a -= b; if(a < 0) a += mod; return a;}
inline int mul(int a, int b){return (int)((long long) a * b % mod);}
inline int modexpo(int a, int b){int res = 1; while(b > 0){ if(b & 1) res = mul(res, a); a = mul(a, a); b /= 2;} return res;}
inline int divide(int a, int b){ return mul(a, modexpo(b, mod - 2));}

clock_t time_p = clock();

void ktj(){
  time_p = clock() - time_p;
  cerr << "Time elapsed : " << (float)(time_p)/CLOCKS_PER_SEC << "\n";
}

void pre(){ // Reset each global variable (esp. for graphs)
  
}

const int N = 25;

int a[N][N], n, m, d[N][N];
int ans = 0;

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

void solveDijkstra(int x, int y){
  for(int i = 0; i < 4; i++){
    int p = x + dx[i];
    int q = y + dy[i];
    if(p >= 1 and p <= n and q >= 1 and q <= m and a[p][q]){
      if(d[p][q] > 1 + d[x][y]){
        d[p][q] = 1 + d[x][y];
        solveDijkstra(p, q);
      }
    }
  }
}

void solve(){
  int i, j, k;
  cin >> n >> m;
  for(i = 1; i <= n; i++){
    string s; cin >> s;
    s = '#' + s;
    for(j = 1; j <= m; j++){
      a[i][j] = (s[j] == '.');
    }
  }

  for(i = 1; i <= n; i++){
    for(j = 1; j <= m; j++){
      if(a[i][j]){
        for(int l = 1; l <= n; l++){
          for(int r = 1; r <= m; r++){
            if(d[l][r] != inf){
              ans = max(ans, d[l][r]);
            }
            d[l][r] = inf;
          }
        }
        d[i][j] = 0;
        solveDijkstra(i, j);
      }
    }
  }
  for(int l = 1; l <= n; l++){
    for(int r = 1; r <= m; r++){
      if(d[l][r] != inf){
        ans = max(ans, d[l][r]);
      }
      d[l][r] = inf;
    }
  }

  cout << ans << '\n';
}

int main(){
  fastio

  pre();

  int t = 1;
  // cin >> t;
  while(t--){
    solve();
  }

  ktj();
}


Measured memory_bytes: 256