#include <bits/stdc++.h>
using namespace std;
using ll = long long;
using P = pair<long long, long long>;
long long dx[] = {1, 0, -1, 0};
long long dy[] = {0, 1, 0, -1};
constexpr ll mod = 1e9 + 7;
constexpr ll inf = INT32_MAX / 2;
constexpr ll INF = LLONG_MAX / 2;
constexpr long double eps = DBL_EPSILON;
constexpr long double pi = 3.141592653589793238462643383279;
template <class T>
inline bool chmin(T& a, T b) {
  if (a > b) {
    a = b;
    return true;
  }
  return false;
}
template <class T>
inline bool chmax(T& a, T b) {
  if (a < b) {
    a = b;
    return true;
  }
  return false;
}
ll fact(long long i) {
  if (i == 0) return 1;
  return (fact(i - 1)) * i % mod;
}
ll gcd(ll a, ll b) {
  if (b == 0) return a;
  return gcd(b, a % b);
}
ll lcm(ll a, ll b) { return a * b / gcd(a, b); }
long long keta(ll n) {
  if (n == 0) return 1;
  long long count = 0;
  while (n != 0) {
    n /= 10;
    count++;
  }
  return count;
}
ll ketasum(ll n) {
  ll sum = 0;
  while (n != 0) {
    sum += n % 10;
    n /= 10;
  }
  return sum;
}
signed main() {
  long long h, w;
  cin >> h >> w;
  vector<vector<bool>> Bo(h, vector<bool>(w, false));
  vector<vector<long long>> v(h, vector<long long>(w, inf));
  for (long long i = 0; i < (long long)(h); i++) {
    for (long long j = 0; j < (long long)(w); j++) {
      char c;
      cin >> c;
      if (c == '#') {
        Bo[i][j] = true;
        v[i][j] = -1;
      }
    }
  }
  long long Max = -1;
  for (long long i = 0; i < (long long)(h); i++) {
    for (long long j = 0; j < (long long)(w); j++) {
      vector<vector<bool>> bo = Bo;
      vector<vector<long long>> ans = v;
      long long ma = -1;
      queue<P> que;
      que.push({i, j});
      ans[i][j] = 0;
      while (!que.empty()) {
        P p = que.front();
        que.pop();
        for (long long k = 0; k < (long long)(4); k++) {
          long long x = p.first + dx[k], y = p.second + dy[k];
          if (x < 0 || x >= h || y < 0 || y >= w || bo[x][y]) continue;
          ans[x][y] = ans[p.first][p.second] + 1;
          chmax(ma, ans[x][y]);
          que.push({x, y});
          bo[x][y] = true;
        }
      }
      chmax(Max, ma);
    }
  }
  cout << Max << endl;
  return 0;
}
