diff --git a/hs/src/Aoc/Y2020.hs b/hs/src/Aoc/Y2020.hs
index ddb03fd..d347d1d 100644
--- a/hs/src/Aoc/Y2020.hs
+++ b/hs/src/Aoc/Y2020.hs
@@ -11,6 +11,7 @@ import qualified Aoc.Y2020.D05 as D05
 import qualified Aoc.Y2020.D06 as D06
 import qualified Aoc.Y2020.D07 as D07
 import qualified Aoc.Y2020.D08 as D08
+import qualified Aoc.Y2020.D09 as D09
 
 year :: Year
 year = Year 2020
@@ -22,4 +23,5 @@ year = Year 2020
   , ( 6, D06.day)
   , ( 7, D07.day)
   , ( 8, D08.day)
+  , ( 9, D09.day)
   ]
diff --git a/hs/src/Aoc/Y2020/D09.hs b/hs/src/Aoc/Y2020/D09.hs
new file mode 100644
index 0000000..2ac0ee8
--- /dev/null
+++ b/hs/src/Aoc/Y2020/D09.hs
@@ -0,0 +1,50 @@
+module Aoc.Y2020.D09
+  ( day
+  ) where
+
+import           Control.Monad
+import           Data.List
+
+import           Aoc.Day
+import           Aoc.Parse
+
+parser :: Parser [Int]
+parser = manyLines decimal
+
+splitAndGroup :: Int -> [Int] -> [([Int], Int)]
+splitAndGroup width
+  = map (\l -> (take width l, l !! width))
+  . filter ((> width) . length)
+  . tails
+
+isValid :: [Int] -> Int -> Bool
+isValid nums n = not $ null $ do
+  a <- nums
+  b <- nums
+  guard $ a /= b
+  guard $ a + b == n
+  pure ()
+
+-- Fast enough to give me a result, but not linear time. I couldn't figure out a
+-- way to implement the linear time algorithm that didn't look like a mess.
+-- Maybe I'll have another go for the cleanup.
+findRange :: Int -> [Int] -> [Int]
+findRange n
+  = head
+  . filter ((== n) . sum)
+  . map (head . dropWhile ((< n) . sum) . inits)
+  . tails
+
+solver :: [Int] -> IO ()
+solver nums = do
+  putStrLn ">> Part 1"
+  let (_, invalidN) = head $ dropWhile (uncurry isValid) $ splitAndGroup 25 nums
+  print invalidN
+
+  putStrLn ""
+  putStrLn ">> Part 2"
+  let weakness = findRange invalidN nums
+  print $ minimum weakness + maximum weakness
+
+day :: Day
+day = dayParse parser solver