Already did it, but the idea is not every user posting a solution in discourse, you can post your solution if it doesn't work following this guideline when requesting some help. Read this and this to help you get how things work here. Welcome and have fun.

Please refrain of posting your solutions in kata's discourse.

Nothing is preimported, why do you need Linq? You can sum numbers without it and if you want to use it, simply import it. Also, mark your post as having spoiler content next time.