def convert_postag(pos):
    """Convert NLTK POS tags to SentiWordNet's POS tags."""
    if pos in ['VB', 'VBD', 'VBG', 'VBN', 'VBP', 'VBZ']:
        return 'v'
    elif pos in ['JJ', 'JJR', 'JJS']:
        return 'a'
    elif pos in ['RB', 'RBR', 'RBS']:
        return 'r'
    elif pos in ['NNS', 'NN', 'NNP', 'NNPS']:
        return 'n'
    else:
        return 'u'