diff --git a/AdventOfCode2025/Inputs/Day9_0.txt b/AdventOfCode2025/Inputs/Day9_0.txt
new file mode 100644
index 0000000..7fb70de
--- /dev/null
+++ b/AdventOfCode2025/Inputs/Day9_0.txt
@@ -0,0 +1,8 @@
+7,1
+11,1
+11,7
+9,7
+9,5
+2,5
+2,3
+7,3
\ No newline at end of file
diff --git a/AdventOfCode2025/Inputs/Day9_1.txt b/AdventOfCode2025/Inputs/Day9_1.txt
new file mode 100644
index 0000000..26e2789
--- /dev/null
+++ b/AdventOfCode2025/Inputs/Day9_1.txt
@@ -0,0 +1,496 @@
+97989,50290
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\ No newline at end of file
diff --git a/AdventOfCode2025/Solutions/Day9.cs b/AdventOfCode2025/Solutions/Day9.cs
new file mode 100644
index 0000000..834a815
--- /dev/null
+++ b/AdventOfCode2025/Solutions/Day9.cs
@@ -0,0 +1,119 @@
+using Position = (long X, long Y);
+
+namespace AdventOfCode2025.Solutions;
+
+public class Day9() : Solution(9)
+{
+    private const char SplitChar = ',';
+
+    public override string SolvePart1() =>
+        GetPositionCombinations(GetPositions().ToList())
+            .Select(combination => CalculateRectangleArea(combination.Pos1, combination.Pos2))
+            .Max()
+            .ToString();
+
+    public override string SolvePart2()
+    {
+        List<Position> positions = GetPositions().ToList();
+
+        // var areas =
+        //     GetPositionCombinations(positions)
+        //         .Select(GetRectangleCorners)
+        //         .Where(corners => corners.All(pos => IsPointInsidePolygon(pos, positions)))
+        //         .Select(corners => CalculateRectangleArea(corners[0], corners[2]))
+        //         .Max();
+        var areas =
+            GetPositionCombinations(positions)
+                .AsParallel()
+                .Where(combination => AreAllRectanglePointsInsidePolygon(combination.Pos1, combination.Pos2, positions))
+                .Select(combination => CalculateRectangleArea(combination.Pos1, combination.Pos2))
+                .Max();
+        return areas.ToString();
+    }
+
+    private static bool AreAllRectanglePointsInsidePolygon(Position pos1, Position pos2, List<Position> polygon)
+    {
+        var minX = Math.Min(pos1.X, pos2.X);
+        var maxX = Math.Max(pos1.X, pos2.X);
+        var minY = Math.Min(pos1.Y, pos2.Y);
+        var maxY = Math.Max(pos1.Y, pos2.Y);
+
+        if (GetRectangleCorners(pos1, pos2).All(corner => IsPointInsidePolygon(corner, polygon)))
+        {
+            return false;
+        }
+
+        for (var x = minX; x <= maxX; x++)
+        {
+            for (var y = minY; y <= maxY; y++)
+            {
+                if (!IsPointInsidePolygon((x, y), polygon))
+                {
+                    return false;
+                }
+            }
+        }
+
+        return true;
+    }
+
+    // ray-casting algorithm or even-odd rule
+    private static bool IsPointInsidePolygon(Position point, List<Position> polygon)
+    {
+        var intersections = 0;
+        var n = polygon.Count;
+
+        for (var i = 0; i < n; i++)
+        {
+            var p1 = polygon[i];
+            var p2 = polygon[(i + 1) % n];
+
+            if (p1.Y > point.Y != p2.Y > point.Y && point.X < (p2.X - p1.X) * (point.Y - p1.Y) / (p2.Y - p1.Y) + p1.X)
+            {
+                intersections++;
+            }
+        }
+
+        return intersections % 2 == 1;
+    }
+
+    private static IEnumerable<(Position Pos1, Position Pos2)> GetPositionCombinations(List<Position> positions)
+    {
+        for (var i = 0; i < positions.Count; i++)
+        {
+            for (var j = i + 1; j < positions.Count; j++)
+            {
+                yield return (positions.ElementAt(i), positions.ElementAt(j));
+            }
+        }
+    }
+
+    private static long CalculateRectangleArea(Position pos1, Position pos2)
+    {
+        var length = Math.Abs(pos1.X - pos2.X) + 1;
+        var width = Math.Abs(pos1.Y - pos2.Y) + 1;
+        return length * width;
+    }
+
+    private static Position[] GetRectangleCorners(Position pos1, Position pos2)
+    {
+        var minX = Math.Min(pos1.X, pos2.X);
+        var maxX = Math.Max(pos1.X, pos2.X);
+        var minY = Math.Min(pos1.Y, pos2.Y);
+        var maxY = Math.Max(pos1.Y, pos2.Y);
+
+        return
+        [
+            (minX, minY),
+            (maxX, minY),
+            (maxX, maxY),
+            (minX, maxY)
+        ];
+    }
+
+    private IEnumerable<Position> GetPositions() => Input.Lines.Select(line =>
+    {
+        var values = line.Split(SplitChar);
+        return (X: long.Parse(values[0]), Y: long.Parse(values[1]));
+    });
+}
\ No newline at end of file