#include <bits/stdc++.h>
using namespace std;
using ll = long long;
template <typename T>
T gcd(T a, T b) {
  if (a < b) return gcd(b, a);
  T r;
  while ((r = a % b)) {
    a = b;
    b = r;
  }
  return b;
}
template <typename T>
T lcm(T a, T b) {
  return a * b / gcd(a, b);
}
template <typename T>
T ext_gcd(T a, T b, T& x, T& y) {
  x = 1;
  y = 0;
  T x0 = 0, y0 = 1;
  while (b != 0) {
    T q = a / b;
    a -= q * b;
    swap(a, b);
    x -= q * x0;
    swap(x, x0);
    y -= q * y0;
    swap(y, y0);
  }
  return a;
}
template <typename T>
T inv_mod(T a, T mod) {
  T t1, t2;
  ext_gcd(a, mod, t1, t2);
  t1 %= mod;
  if (t1 < 0) t1 += mod;
  return t1;
}
template <typename T>
T combination_large(T n, T k, T mod) {
  if (n < k || n < 0 || k < 0) return 0;
  vector<T> inv(n + 1, 1), fact(n + 1, 1), fact_inv(n + 1, 1);
  for (T i = 2; i <= n; ++i) {
    inv[i] = mod - inv[mod % i] * (mod / i) % mod;
    fact[i] = (fact[i - 1] * i) % mod;
    fact_inv[i] = fact_inv[i - 1] * inv[i] % mod;
  }
  return fact[n] * (fact_inv[k] * fact_inv[n - k] % mod) % mod;
}
template <typename T>
T combination(T n, T k, T mod = 1) {
  vector<vector<T>> table(n + 1, vector<T>(k + 1, 1));
  for (T i = 1; i <= n; ++i) {
    table[i][0] = 1;
    for (T j = 1; j <= n; ++j) {
      table[i][j] = (table[i - 1][j - 1] + table[i - 1][j]) % mod;
    }
  }
}
int main(int, char**) {
  int H, W;
  cin >> H >> W;
  cin.ignore();
  vector<vector<bool>> S(H, vector<bool>(W));
  for (int i = 0; i < H; ++i) {
    for (int j = 0; j < W; ++j) {
      char tmp;
      cin >> tmp;
      S[i][j] = (tmp == '.');
    }
    cin.ignore();
  }
  int max_cost = 0;
  for (int sy = 0; sy < H; ++sy) {
    for (int sx = 0; sx < W; ++sx) {
      if (!S[sy][sx]) continue;
      for (int gy = 0; gy < H; ++gy) {
        for (int gx = 0; gx < W; ++gx) {
          if (!S[gy][gx]) continue;
          deque<pair<int, int>> q;
          vector<int> costs(H * W, INT_MAX);
          q.push_back(make_pair(sy * W + sx, 0));
          while (!q.empty()) {
            auto p = q.front();
            q.pop_front();
            if (costs[p.first] < p.second || !S[p.first / W][p.first % W])
              continue;
            costs[p.first] = p.second;
            if (p.first == gy * W + gx) {
              max_cost = max(max_cost, p.second);
              break;
            }
            if (p.first % W != 0)
              q.push_back(make_pair(p.first - 1, p.second + 1));
            if (p.first % W != W - 1)
              q.push_back(make_pair(p.first + 1, p.second + 1));
            if (p.first / W != 0)
              q.push_back(make_pair(p.first - W, p.second + 1));
            if (p.first / W != H - 1)
              q.push_back(make_pair(p.first + W, p.second + 1));
          }
        }
      }
    }
  }
  cout << max_cost << endl;
  return 0;
}
