#include <bits/stdc++.h>
using namespace std;
using ll = int_fast64_t;
using v_b = vector<bool>;
using v_ll = vector<ll>;
using str = string;
using v_str = vector<string>;
using p_ll = pair<ll, ll>;
using vv_b = vector<v_b>;
using vv_ll = vector<v_ll>;
using vp_ll = vector<p_ll>;
using vvv_ll = vector<vv_ll>;
using vvp_ll = vector<vp_ll>;
using ld = long double;
using v_ld = vector<ld>;
using vv_ld = vector<v_ld>;
bool IS_TEST = false;
ll ll_min64 = 1LL << 63;
ll ll_max64 = ~ll_min64;
ll ll_min32 = 1LL << 31;
ll ll_max32 = ~ll_min32;
ll MOD = 1000000007;
template <class T>
void show2(const T& x) {
  cout << x;
}
template <class T1, class T2>
void show2(const pair<T1, T2>& x) {
  cout << "{" << show2(x.first) << "," << show2(x.second) << "}";
}
template <class T>
void show(const T& x) {
  if (!IS_TEST) return;
  show2(x);
  cout << endl;
}
template <class T>
void v_show(const T& v, ll n = -1) {
  if (!IS_TEST) return;
  auto itr = v.begin();
  ll m = n;
  while (itr != v.end() && m != 0) {
    show2(*itr);
    cout << " ";
    itr++;
    m--;
  }
  cout << endl;
}
template <class T>
void vv_show(const T& v, ll n = -1) {
  if (!IS_TEST) return;
  cout << "--------------------------------\n";
  auto itr = v.begin();
  ll m = n;
  while (itr != v.end() && m != 0) {
    v_show(*itr, n);
    itr++;
    m--;
  }
  cout << "--------------------------------" << endl;
}
void load(ll& x1) { cin >> x1; }
void load(ll& x1, ll& x2) { cin >> x1 >> x2; }
void load(ll& x1, ll& x2, ll& x3) { cin >> x1 >> x2 >> x3; }
void load(ll& x1, ll& x2, ll& x3, ll& x4) { cin >> x1 >> x2 >> x3 >> x4; }
void v_load(ll n, v_ll& v1, ll head = 0, ll tail = 0, ll init = 0) {
  ll m = n + head + tail;
  v1.assign(m, init);
  for (ll i = 0; i < n; i++) {
    cin >> v1[i + head];
  }
}
void v_load(ll n, v_ll& v1, v_ll& v2, ll head = 0, ll tail = 0, ll init = 0) {
  ll m = n + head + tail;
  v1.assign(m, init);
  v2.assign(m, init);
  for (ll i = 0; i < n; i++) {
    cin >> v1[i + head] >> v2[i + head];
  }
}
void v_load(ll n, v_ll& v1, v_ll& v2, v_ll& v3, ll head = 0, ll tail = 0,
            ll init = 0) {
  ll m = n + head + tail;
  v1.assign(m, init);
  v2.assign(m, init);
  v3.assign(m, init);
  for (ll i = 0; i < n; i++) {
    cin >> v1[i + head] >> v2[i + head] >> v3[i + head];
  }
}
void v_load(ll n, v_ll& v1, v_ll& v2, v_ll& v3, v_ll& v4, ll head = 0,
            ll tail = 0, ll init = 0) {
  ll m = n + head + tail;
  v1.assign(m, init);
  v2.assign(m, init);
  v3.assign(m, init);
  v4.assign(m, init);
  for (ll i = 0; i < n; i++) {
    cin >> v1[i + head] >> v2[i + head] >> v3[i + head] >> v4[i + head];
  }
}
v_ll ll_mg(ll x1 = ll_max64, ll x2 = ll_max64, ll x3 = ll_max64,
           ll x4 = ll_max64) {
  v_ll x{x1, x2, x3, x4};
  sort(x.begin(), x.end());
  return x;
}
template <class T>
void ch_max(T& x, const T& y) {
  if (x >= y) return;
  x = y;
}
template <class T>
void ch_min(T& x, const T& y) {
  if (x <= y) return;
  x = y;
}
template <class T, class S>
void ch_max(T& x, S& xx, const T& y, const S& yy) {
  if (x >= y) return;
  x = y;
  xx = yy;
}
template <class T, class S>
void ch_min(T& x, S& xx, const T& y, const S& yy) {
  if (x <= y) return;
  x = y;
  xx = yy;
}
template <class T>
void quit(T x) {
  cout << x << endl;
  exit(0);
}
ll rem(ll x, ll y) {
  ll z = x % y;
  return z >= 0 ? z : z + y;
}
ll H, W;
v_str S;
int main() {
  load(H, W);
  S.assign(H, "");
  for (ll i = 0; i < H; i++) {
    cin >> S[i];
  }
  ll x = 0;
  for (ll i1 = 0; i1 < H; i1++) {
    for (ll j1 = 0; j1 < W; j1++) {
      queue<p_ll> Q;
      vv_ll M(H, v_ll(W, -1));
      M[i1][j1] = 0;
      Q.push(p_ll(i1, j1));
      while (!Q.empty()) {
        p_ll f = Q.front();
        Q.pop();
        if (f.first - 1 >= 0) {
          if (M[f.first - 1][f.second] < 0) {
            if (S[f.first - 1][f.second] == '.') {
              M[f.first - 1][f.second] = M[f.first][f.second] + 1;
              Q.push(p_ll(f.first - 1, f.second));
            }
          }
        }
        if (f.first + 1 < H) {
          if (M[f.first + 1][f.second] < 0) {
            if (S[f.first + 1][f.second] == '.') {
              M[f.first + 1][f.second] = M[f.first][f.second] + 1;
              Q.push(p_ll(f.first + 1, f.second));
            }
          }
        }
        if (f.second - 1 >= 0) {
          if (M[f.first][f.second - 1] < 0) {
            if (S[f.first][f.second - 1] == '.') {
              M[f.first][f.second - 1] = M[f.first][f.second] + 1;
              Q.push(p_ll(f.first, f.second - 1));
            }
          }
        }
        if (f.second + 1 < W) {
          if (M[f.first][f.second + 1] < 0) {
            if (S[f.first][f.second + 1] == '.') {
              M[f.first][f.second + 1] = M[f.first][f.second] + 1;
              Q.push(p_ll(f.first, f.second + 1));
            }
          }
        }
      }
      for (ll i = 0; i < H; i++) {
        for (ll j = 0; j < W; j++) {
          ch_max(x, M[i][j]);
        }
      }
    }
  }
  cout << x << endl;
}
