#include <bits/stdc++.h>
using namespace std;
vector<int> x = {0, 1, 0, -1};
vector<int> y = {1, 0, -1, 0};
queue<vector<int>> q;
int main() {
  int H, W, length, maxLen;
  string s;
  cin >> H >> W;
  length = maxLen = 0;
  vector<vector<int>> map(H, vector<int>(W, 0));
  queue<vector<int>> q;
  for (int i = (int)(0); i < (int)(H); i++) {
    cin >> s;
    for (int j = (int)(0); j < (int)(s.size()); j++) {
      if (s[j] == '.') {
        map[i][j] = 1;
      }
    }
  }
  for (int s = (int)(0); s < (int)(H * W); s++) {
    for (int g = (int)(s); g < (int)(H * W); g++) {
      int si = s / W;
      int sj = s % W;
      int gi = g / W;
      int gj = g % W;
      if (map[si][sj] == 1 && map[gi][gj] == 1) {
        vector<vector<int>> memo(H, vector<int>(W, 0));
        memo[si][sj] = 1;
        length = 0;
        q.push({si, sj});
        while (!q.empty()) {
          vector<int> cur = q.front();
          q.pop();
          if (cur[0] == gi && cur[1] == gj) {
            length = memo[cur[0]][cur[1]] - 1;
          }
          for (int i = (int)(0); i < (int)(4); i++) {
            int next_y = cur[0] + y[i];
            int next_x = cur[1] + x[i];
            if (next_y >= 0 && next_x >= 0 && next_y < H && next_x < W) {
              if (map[next_y][next_x] == 1 && !memo[next_y][next_x]) {
                q.push({next_y, next_x});
                memo[next_y][next_x] = memo[cur[0]][cur[1]] + 1;
              }
            }
          }
        }
      }
      if (maxLen < length) {
        maxLen = length;
      }
    }
  }
  cout << maxLen << endl;
}
