def num_speakers(speaker_lists):
    """
    Computes number of speakers in a document.
    """
    flat_speaker_list = [speaker for speaker_list in speaker_lists for speaker in speaker_list]
    speaker_set = set(flat_speaker_list)
    speaker_set.discard("[SPL]")
    speaker_set.discard("-")
    return max(1, len(speaker_set))