#include <bits/stdc++.h>
using namespace std;
int INF = 1e9 + 7;
struct edge {
  int to;
  int cost;
};
int V;
vector<edge> G[300];
int d[300][300];
void warshall_floyd() {
  for (int i = 0; i < V; i++) {
    fill(d[i], d[i] + V, INF);
    d[i][i] = 0;
    for (int j = 0; j < G[i].size(); j++) {
      edge e = G[i][j];
      d[i][e.to] = e.cost;
    }
  }
  for (int k = 0; k < V; k++) {
    for (int i = 0; i < V; i++) {
      for (int j = 0; j < V; j++) {
        d[i][j] = min(d[i][j], d[i][k] + d[k][j]);
      }
    }
  }
}
int main() {
  int h, w;
  cin >> h >> w;
  int s[h][w];
  V = 0;
  for (int i = 0; i < h; i++) {
    for (int j = 0; j < w; j++) {
      char c;
      cin >> c;
      if (c == '.') {
        s[i][j] = V++;
      } else {
        s[i][j] = -1;
      }
    }
  }
  for (int i = 0; i < h; i++) {
    for (int j = 0; j < w; j++) {
      if (s[i][j] != -1) {
        edge e;
        e.cost = 1;
        if (i - 1 >= 0 && s[i - 1][j] != -1) {
          e.to = s[i - 1][j];
          G[s[i][j]].push_back(e);
        }
        if (i + 1 < h && s[i + 1][j] != -1) {
          e.to = s[i + 1][j];
          G[s[i][j]].push_back(e);
        }
        if (j - 1 >= 0 && s[i][j - 1] != -1) {
          e.to = s[i][j - 1];
          G[s[i][j]].push_back(e);
        }
        if (j + 1 < w && s[i][j + 1] != -1) {
          e.to = s[i][j + 1];
          G[s[i][j]].push_back(e);
        }
      }
    }
  }
  warshall_floyd();
  int ans = 0;
  for (int i = 0; i < V; i++) {
    for (int j = 0; j < V; j++) {
      ans = max(d[i][j], ans);
    }
  }
  cout << ans << endl;
  return 0;
}
