class AdvancedIterator:
    
    def __init__(self,sequence):
        self.sequence=sequence
        self.index=-1
        
    def _next(self):
        try:
            return self.sequence[self.index+1]
        except IndexError:
            raise StopIteration
    
    def _previous(self):
        if self.index<0:
            raise StopIteration
        
        return self.sequence[self.index]
    
    def next(self):
        item=self._next()
        self.index+=1
        
        return item
    
    def previuos(self):
        item=self._previous()
        self.index-=1
        
        return item