#include <bits/stdc++.h>
using namespace std;
const int maxn = 105;
const int dx[] = {-1, 0, 1, 0}, dy[] = {0, -1, 0, 1};
int n, m;
int a[maxn][maxn], d[maxn][maxn];
char s[maxn];
int main() {
  scanf("%d%d", &n, &m);
  memset(a, -1, sizeof(a));
  for (int i = 1; i <= n; ++i) {
    scanf("%s", s + 1);
    for (int j = 1; j <= m; ++j) {
      a[i][j] = (s[j] == '#');
    }
  }
  int ans = 0;
  queue<int> q;
  for (int i = 1; i <= n; ++i) {
    for (int j = 1; j <= m; ++j) {
      memset(d, -1, sizeof(d));
      d[i][j] = 0;
      q.push(i);
      q.push(j);
      while (!q.empty()) {
        int x = q.front();
        q.pop();
        int y = q.front();
        q.pop();
        for (int k = 0; k < 4; ++k) {
          int xx = x + dx[k], yy = y + dy[k];
          if (a[xx][yy] == 0 && d[xx][yy] == -1) {
            q.push(xx);
            q.push(yy);
            d[xx][yy] = d[x][y] + 1;
          }
        }
      }
      for (int x = 1; x <= n; ++x) {
        for (int y = 1; y <= m; ++y) {
          ans = max(ans, d[x][y]);
        }
      }
    }
  }
  printf("%d\n", ans);
  return 0;
}
