import itertools
H,W= [int(x) for x in input().split()]
meiro=[list(input())for _ in range(H)]
#print(meiro)
ans=0
dx = [1,0,-1,0]
dy=[0,1,0,-1]
for i in range(H):
  for j in range(W):
    result=[[0]*W for _ in range(H)]
    cnt=0
    #print(i,j)
    if meiro[i][j]=="#":
      continue
    elif meiro[i][j]==".":
      stk =[[i,j]]
      result[i][j]=1
      while stk:
        x,y = stk.pop(0)
        
        for r in range(4):
          xx,yy=x+dx[r],y+dy[r]
          if yy<0 or xx<0 or W<=yy or H<=xx or meiro[xx][yy]=="#" or result[xx][yy]!=0:
            continue
          result[xx][yy]=result[x][y]+1
          stk.append([xx,yy])
      for l in range(H):
        for t in range(W):
          ans = max(ans,result[l][t])
print(ans-1)import itertools
H,W= [int(x) for x in input().split()]
meiro=[list(input())for _ in range(H)]
#print(meiro)
ans=0
dx = [1,0,-1,0]
dy=[0,1,0,-1]
for i in range(H):
  for j in range(W):
    result=[[0]*W for _ in range(H)]
    cnt=0
    #print(i,j)
    if meiro[i][j]=="#":
      continue
    elif meiro[i][j]==".":
      stk =[[i,j]]
      result[i][j]=1
      while stk:
        x,y = stk.pop(0)
        
        for r in range(4):
          xx,yy=x+dx[r],y+dy[r]
          if yy<0 or xx<0 or W<=yy or H<=xx or meiro[xx][yy]=="#" or result[xx][yy]!=0:
            continue
          result[xx][yy]=result[x][y]+1
          stk.append([xx,yy])
      for l in range(H):
        for t in range(W):
          ans = max(ans,result[l][t])
print(ans-1)