from collections import deque
import numpy as np
 
 
H,W= [int(x) for x in input().split()]
n=[]
T=np.zeros((H,W),dtype=int)
T=[]
S=np.zeros_like(T,dtype=int) 
for i in range(H):
  T[i]=[x for x in input().split()]
#B=T.copy()    
d=0
c=deque()  
for i in range(H):
    for j in range(W):
         
        if T[i][j] =="#":continue
 
          
        c.append((i,j))
        S[i][j] = 1
        #c.append(j)
        
        #t=len(c)
        while True:

                try:
                  m,n = c.popleft()
                except IndexError: break
                
                
                for kk in [(m+1,n),(m-1,n),(m,n+1),(m,n-1)]:
                  if not (0<=kk[0]<H and 0<= kk[1] <W): continue
                  if T[kk]=="#":continue
                  if S[kk] == 0:
                    c.append(kk)
                    S[kk] = S[m][n]+1        
                        
 
        #print(T)
        d = max(d,np.max(S))
        S=np.zeros_like(T,dtype=int)
        
print(d-1)
