diff --git a/app/Day9.hs b/app/Day9.hs
new file mode 100644
index 0000000..6f61681
--- /dev/null
+++ b/app/Day9.hs
@@ -0,0 +1,55 @@
+module Main where
+
+import Data.List (uncons)
+import Data.Maybe (mapMaybe)
+import Data.Set (Set)
+import Data.Set qualified as S
+import System.Environment (getArgs)
+
+main :: IO ()
+main = do
+  input <- maybe (error "Missing argument to input file") (readFile . fst) =<< uncons <$> getArgs
+  putStrLn $ unwords ["Part 1:", show $ part1 input]
+  putStrLn $ unwords ["Part 2:", show $ part2 input]
+
+area :: (Int, Int) -> (Int, Int) -> Int
+area (x1, y1) (x2, y2) = x' * y'
+  where
+    x' = max x1 x2 - min x1 x2 + 1
+    y' = max y1 y2 - min y1 y2 + 1
+
+parse :: String -> [(Int, Int)]
+parse = mapMaybe (fmap fst . uncons . go) . lines
+  where
+    go :: String -> [(Int, Int)]
+    go s = do
+      (x, ',' : s1) <- reads s
+      (y, []) <- reads s1
+      pure (x, y)
+
+combine :: (Ord a) => [a] -> [(a, a)]
+combine as = S.toList $ foldl' (go as) S.empty as
+  where
+    go :: (Ord a) => [a] -> Set (a, a) -> a -> Set (a, a)
+    go as' pairs b = foldl' (\pairs' a -> if a /= b then S.insert (a, b) pairs' else pairs') pairs as'
+
+-- | Idea: Generate pairs of all coordinates and calculate the area of
+-- them. Take the maximum.
+part1 :: String -> Int
+part1 = maximum . map (uncurry area) . combine . parse
+
+part2 :: String -> Int
+part2 = error "Not implemented"
+
+testInput :: String
+testInput =
+  unlines
+    [ "7,1",
+      "11,1",
+      "11,7",
+      "9,7",
+      "9,5",
+      "2,5",
+      "2,3",
+      "7,3"
+    ]
diff --git a/input/day9.txt b/input/day9.txt
new file mode 100644
index 0000000..45aaae8
--- /dev/null
+++ b/input/day9.txt
@@ -0,0 +1,496 @@
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