def load_start_time(df_start_time, vid_name):
    """
    load start time.
    Args:
        df_start_time: dataframe, start time dataframe
        vid_name: str, video name

    Returns:
        int
    """
    if vid_name not in df_start_time.index:
        return None
    start_time = df_start_time.loc[vid_name]["start_time"]
    return int(start_time)