21         
            
 
            
 
            
 
            
 
            
 
            
 
            
 
            
 
            
 
            
 
            
 
            
 
            
 
            
 
            
 
            
 
            
 
            
 
            
 
            
 
            
 
            
 
            
 
            
 
            
 
            
 
            
 
            
 
            
 
            
18, 19, 20         
            
 
            
 
            
18.5