Add project files.

Day1/Day1.csproj

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+<Project Sdk="Microsoft.NET.Sdk">
+
+  <PropertyGroup>
+    <OutputType>Exe</OutputType>
+    <TargetFramework>net8.0</TargetFramework>
+    <ImplicitUsings>enable</ImplicitUsings>
+    <Nullable>enable</Nullable>
+  </PropertyGroup>
+
+  <ItemGroup>
+    <None Update="input.txt">
+      <CopyToOutputDirectory>Always</CopyToOutputDirectory>
+    </None>
+  </ItemGroup>
+
+</Project>

Day1/Program.cs

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+var lines = File.ReadAllLines("input.txt");
+
+var parts = lines.Select(line => (line.Split("   ")[0], line.Split("   ")[1])).ToList();
+var list1 = parts.Select(part => int.Parse(part.Item1)).Order().ToList();
+var list2 = parts.Select(part => int.Parse(part.Item2)).Order().ToList();
+int distance = 0, similarityScore = 0;
+
+for (var i = 0; i < list1.Count; i++)
+{
+    distance += Math.Abs(list1[i] - list2[i]);
+    similarityScore += list1[i] * list2.Count(item => item == list1[i]);
+}
+
+Console.WriteLine($"Part 1 : {distance}\nPart 2 : {similarityScore}");