After saving Christmas five years in a row, you've decided to take a vacation at a nice resort on a tropical island. Surely, Christmas will go on without you.

The tropical island has its own currency and is entirely cash-only. The gold coins used there have a little picture of a starfish; the locals just call them stars. None of the currency exchanges seem to have heard of them, but somehow, you'll need to find fifty of these coins by the time you arrive so you can pay the deposit on your room.

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using System.Collections.Generic;
using System.Linq;

namespace AdventOfCode.Y2020.Day01;

[ProblemName("Report Repair")]
class Solution : Solver {

    public object PartOne(string input) {
        var numbers = Numbers(input);
        return (
            from x in numbers 
            let y = 2020 - x
            where numbers.Contains(y)
            select x * y
        ).First();
    }

    public object PartTwo(string input) {
        var numbers = Numbers(input);
        return (
            from x in numbers 
            from y in numbers 
            let z = 2020 - x - y
            where numbers.Contains(z)
            select x * y * z
        ).First();
    }

    HashSet<int> Numbers(string input) {
        return input.Split('\n').Select(int.Parse).ToHashSet<int>();
    }
}

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