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)
 
for i in range(H):
  n=input()
  for j in range(W):
    if n[j]=="#": T[i][j]=1
B=T.copy()    
d=0
c=deque() 
for i in range(H):
    for j in range(W):
         
        if T[i][j] ==1:continue

        s=0
        c+=[i,j]
        t=len(c)//2
        while t>0:
                s+=1
                t=len(c)//2
                for k in range(t):
                        m = c.popleft()
                        n = c.popleft()
                        T[m][n]=s
                        if m != H-1:
                                if T[m+1][n]==0:c+=[m+1,n]
                        if m != 0:
                                if T[m-1][n]==0:c+=[m-1,n] 
                        if n != W-1:
                                if T[m][n+1]==0:c+=[m,n+1]
                        if n != 0:
                                if T[m][n-1]==0:c+=[m,n-1]
                        #c += (chk_route([c.popleft(),c.popleft()]))
                
        #print(T)
        d = max(d,s)
        T=B.copy()
        
print(d-2)