#include <stdio.h>
#include <queue>
using namespace std;

int n,m; char map[22][22]; int d[22][22];
int bfs(int sx, int sy)
{
for (int i=0;i<n;i++) for (int j=0;j<m;j++) d[i][j] = -1;
queue<pair<int, int> > q; q.push({sx,sy}); d[sx][sy] = 0;
  int r = 0;
  while (!q.empty()){
int x = q.front().first, y = q.front().second; r = d[x][y]; q.pop();
    int dx[4] = {0,1,0,-1};
    int dy[4] = {1,0,-1,0};
    for(int k=0;k<4;k++){
int px = x + dx[k], py = y + dy[k];
      if (px < 0 || px >= n || py < 0 || py >= m || map[px][py] == '#') continue;
      if (d[px][py] == -1){
q.push({px,py}); d[px][py] = d[x][y] + 1;
      }
    }
  }
  return r;
}

int main()
{
scanf ("%d %d",&n,&m);
  for (int i=0;i<n;i++) scanf ("%s",map[i]);
  int ans = 0;
  for (int i=0;i<n;i++) for (int j=0;j<m;j++) if (map[i][j] == '.'){
int a = bfs(i,j);
    if (ans < a) ans = a;
  }
  printf ("%d\n",ans);
  return 0;
}
