def n_palavras_unicas(lista_palavras):
    """
        This function receives a list of words and returns the 
        number of words that appear only once.
    """
    freq = dict()
    unicas = 0
    for palavra in lista_palavras:
        p = palavra.lower()
        if p in freq:
            if freq[p] == 1:
                unicas -= 1
            freq[p] += 1
        else:
            freq[p] = 1
            unicas += 1

    return unicas