def format_nums(start_index):
    """
    Expects an integer.
    Brings numbers exceeding three digits in line with API call requirements
    """
    if start_index >= 1000:
        start_format = str(start_index)[:-3] + "%2C" + str(start_index)[-3:]
    else:
        start_format = str(start_index)
    return(start_format)