#include <bits/stdc++.h>
using namespace std;
const int INF = 10000000;
const int dx[] = {1, 0, -1, 0};
const int dy[] = {0, 1, 0, -1};
int main() {
  cin.tie(0);
  ios::sync_with_stdio(false);
  int h, w;
  cin >> h >> w;
  char s[30][30];
  int dist[30][30];
  for (int i = 0; i < h; i++) {
    for (int j = 0; j < w; j++) {
      cin >> s[i][j];
      dist[i][j] = -1;
    }
  }
  int sy, sx, gy, gx;
  int ans = 0;
  for (int i = 0; i < h; i++) {
    for (int j = 0; j < w; j++) {
      for (int k = 0; k < h; k++) {
        for (int l = 0; l < w; l++) {
          sy = i;
          sx = j;
          dist[sy][sx] = 0;
          gy = k;
          gx = l;
          queue<pair<int, int> > que;
          que.push(pair<int, int>(sy, sx));
          while (!que.empty()) {
            pair<int, int> p = que.front();
            que.pop();
            if (p.first == gy && p.second == gx) break;
            for (int m = 0; m < 4; m++) {
              int ny = p.first + dy[m];
              int nx = p.second + dx[m];
              if (0 <= ny && ny < h && 0 <= nx && nx < w && s[ny][nx] != '#' &&
                  dist[ny][nx] == -1) {
                que.push(pair<int, int>(ny, nx));
                dist[ny][nx] = dist[p.first][p.second] + 1;
              }
            }
          }
          ans = max(ans, dist[gy][gx]);
          for (int n = 0; n < h; n++) {
            for (int o = 0; o < w; o++) {
              dist[n][o] = -1;
            }
          }
        }
      }
    }
  }
  cout << ans << endl;
  return 0;
}
