h,w=map(int,input().split())
l=[list(input()) for i in range(h)]
connection=[[] for i in range(h*w)]

for i in range(h):
  for j in range(w):
    if l[i][j]=='.':
      if i!=0:
        if l[i-1][j]!=1:
          connection[i*w+j].append((i-1)*w+j)
      if i!=h-1:
        if l[i+1][j]!=1:
          connection[i*w+j].append((i+1)*w+j)
      if j!=0:
        if l[i][j-1]!=1:
          connection[i*w+j].append(i*w+j-1)
      if j!=w-1:
        if l[i][j+1]!=1:
          connection[i*w+j].append(i*w+j+1)

n=h*w
def bfs(v):
  distance=[-1]*n
  distance[v]=0
  next=connection[v]
  next2=set()
  visited=[-1]*n
  visited[v]=1
  visitct=1
  ct=0
  while len(next)!=0 and visitct!=n:
    ct+=1
    for i in range(len(next)):
      if visited[next[i]]==-1:
        distance[next[i]]=ct
        visited[next[i]]=1
        visitct+=1
        for j in range(len(connection[next[i]])):
          if visited[connection[next[i]][j]]==-1:
            next2.add(connection[next[i]][j])
    next=list(next2)
    next2=set()
  return distance

ans=[]
for i in range(h*w):
  ans.append(max(bfs(i)))
print(max(ans))