def find_longest_annotations(csv_file, n=1):
    """
    Find length and row-index of the N longest annotation windows.
    """
    l_dict = {}
    for rdx in range(len(csv_file)):
        t_min,t_max = csv_file.iloc[rdx].loc[['t_min','t_max']]
        l = t_max - t_min
        l_dict[rdx] = l
    return sorted(list(l_dict.items()), key=lambda x: x[1], reverse=True)[:n]