import numpy as np
from copy import deepcopy
from queue import Queue
temp=list(map(int, input().split()))
R=temp[0]
C=temp[1]

color=np.zeros((R,C)).astype(int)

for i in range(R):
  temp=input()
  a=np.zeros(C).astype(int)
  for j in range(C):
      if temp[j]=='#':
        a[j]=1
      else:
        a[j]=0
  color[i]=a
  
def Adj(u):
  adj=[]
  for v in [(u[0]-1,u[1]),(u[0]+1,u[1]),(u[0],u[1]-1),(u[0],u[1]+1)]:
      if v[0]>=0 and v[0]<R and v[1]>=0 and v[1]<C:
        adj.append(v)
  return adj  

def BFS(s,c,d):
  Q=Queue()
  Q.put(s)
  while not Q.empty():
    u=Q.get()
    for v in Adj(u):  
      if c[v[0],v[1]]==0:
        c[v[0],v[1]]=1
        d[v[0],v[1]]=d[u[0],u[1]]+1
        Q.put(v)
        
def main():
  ma=0
  for sx in range(R):
    for sy in range(C):
      if color[sx,sy]==0:
        d=np.zeros((R,C)).astype(int)
        c=deepcopy(color)
        BFS((sx,sy),c,d)
        if np.max(d)>ma:
          ma=np.max(d)
  print(ma)
  
main()