diff --git a/AOC.Test/Test2015.cs b/AOC.Test/Test2015.cs
index 640e519..1fec09c 100644
--- a/AOC.Test/Test2015.cs
+++ b/AOC.Test/Test2015.cs
@@ -28,6 +28,7 @@ public class Test2015
     [DataRow(typeof(Day20), "665280", "705600")]
     [DataRow(typeof(Day21), "78", "148")]
     [DataRow(typeof(Day23), "255", "334")]
+    [DataRow(typeof(Day25), "9132360", "")]
     public void CheckAllDays(Type dayType, string part1, string part2)
     {
         Common.CheckDay(dayType, part1, part2);
diff --git a/AOC2015/Day25.cs b/AOC2015/Day25.cs
index 54785b8..671f102 100644
--- a/AOC2015/Day25.cs
+++ b/AOC2015/Day25.cs
@@ -3,13 +3,28 @@ namespace AOC2015;
 /// <summary>
 /// Day 25: <a href="https://adventofcode.com/2015/day/25"/>
 /// </summary>
-public sealed class Day25() : Day(2015, 25, "Let It Snow")
+public sealed partial class Day25() : Day(2015, 25, "Let It Snow")
 {
+    [GeneratedRegex(@"\d+")]
+    private static partial Regex NumbersRegex();
+
+    private int _row, _col;
+
     public override void ProcessInput()
     {
+        var s = NumbersRegex().Matches(Input.First()).Select(m => int.Parse(m.Value)).ToList();
+        _row = s[0];
+        _col = s[1];
     }
 
-    public override object Part1() => "";
+    public override object Part1()
+    {
+        var index = _row + _col - 2;
+        index = index * (index + 1) / 2 + _col - 1;
+
+        return Enumerable.Range(0, index)
+            .Aggregate(20151125ul, (current, _) => current * 252533 % 33554393);
+    }
 
     public override object Part2() => "";
 }