def get_most_popular_tracks(urm):
    """
        Returns a pandas dataframe with:
            index:
                - track_id
            columns:
                - track_id
                - its popularity
        They are in in descending order according to their popularity.
        The URM is the matrix with pairs "playlist_id"x"track_id" (i.e. train_final.csv)
    """
    res = urm.groupby('track_id').count().sort_values('playlist_id', ascending=False)
    res['track_id'] = res.index
    return res.rename(columns={'playlist_id': 'count'})