type Birds = Int
type Pole = (Birds, Birds)

landLeft :: Birds -> Pole -> Maybe Pole  
landLeft n (left,right)      
| abs ((left + n) - right) < 4 = Just (left + n, right)      
| otherwise                    = Nothing  

landRight :: Birds -> Pole -> Maybe Pole  landRight n (left,right)      
| abs (left - (right + n)) < 4 = Just (left, right + n)      
| otherwise                    = Nothing 