bg
bg
    8·18       
  

    8    18         8·18                                   
      8    20                                                
             
  
  
  
          
          
            
          
  
  
  
  
  
  
  
  
  
  
  
            
  
  
  
  
  
           
           
  
  
  
         
  
  
  
          
  
  
  
  
         
  
bg
bg