def day15_1():
    with open("input15.txt", "r") as file:
        data = file.read()

    data = [int(n) for n in data.split(",")][::-1]

    while len(data) < 2020:
        n = data[0]

        try:
            age = data.index(n, 1)
        except ValueError:
            age = 0

        data.insert(0, age)

    return data[0]