import re


def parse_countdown_start(arg_array): # throws ValueError
    """Parses the string for starting a countdown. Returns a list with length[, reaction_count]"""

    if len(arg_array) == 0:
        return [10]

    args = re.sub("\s", "", "".join(arg_array)) # join and remove whitespace
    # reaction-type
    if "," in args:
        time, _, reaction_count = args.partition(",")
        return [int(time), int(reaction_count)]
    else:
        return [int(args)]