# [Python 3], 138 bytes

<!-- language-all: lang-python -->

    def g(s,l):n=len(s);u=[i for i in l if i[:n]not in{*l}-{i}and i[:n]==s];return len(u)<2and~(len(u[0])-n)%2or 1-min(g(i[:n+1],l)for i in u)

[Try it online!][TIO-j6ns6oiu]

[Python 3]: https://docs.python.org/3/
[TIO-j6ns6oiu]: https://tio.run/##bY3NasNADITvfgqxUNht41CnN6f7JGYPDtl1Baps9udQQvLqjpyEFFpfJPTNaGb6yV8jf8zz0QcYdNqQadmSZ53MvtgOIYwREJCBAANg17LjMQs4vdK5PuG55@MdW5vcPvpcopgloZjPnYgXfTu6d2dqNi87yWvqb2Q96OXtrXFS@mwpZiawoJSq@sNhzJXvBx@XSf4@FxRDoWVF5KES7zZNhJmQfdKma1pXTSJlqVBqA2TM793/BRK6gug/iysorLCHb74C "Python 3 – Try It Online"