Added implementation of the addEven function

[Karol-Debski]

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[ziobron]

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[ziobron]

@Karol-Debski, we do not have your data in the Coders School database.

1. Please fill in this form https://forms.gle/rkzLsMUSBdnGQgdM9
2. Edit this PR to run this automation once again (you can just add a space in the description or title).
3. Wait one minute. If any label was added to this PR (like Standard, Premium, Podstawy C++, STL) then everything is fine :)
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