def time_string(n):
    """
        Returns a string descibing the corresponding number of days, hours,
        minutes and second to n seconds. Omits '0 days', '0 hours', '0 minutes'
    """
    quantity_days = int(n) // 86400
    n = int(n) % 86400
    quantity_hours = int(n) // 3600
    n = int(n) % 3600
    quantity_minutes = int(n) // 60
    quantity_seconds = int(n) % 60 

    string_days = ''
    string_hours = ''
    string_minutes = ''
    string_seconds = ''
    if quantity_days > 1:
        string_days = str(quantity_days) + ' days, '
    elif quantity_days == 1:
        string_days = '1 day, '
    if quantity_hours > 1:
        string_hours = str(quantity_hours) + ' hours, '
    elif quantity_hours == 1:
        string_hours = '1 hour, '
    if quantity_minutes> 1:
        string_minutes = str(quantity_minutes) + ' minutes, '
    elif quantity_minutes == 1:
        string_minutes = '1 minute, '
    if quantity_seconds == 1:
        string_seconds = '1 second'
    else:
        string_seconds = str(quantity_seconds) + ' seconds'
    
    return string_days + string_hours + string_minutes + string_seconds