diff --git a/hs/src/Aoc/Y2020.hs b/hs/src/Aoc/Y2020.hs
index 880ca75..9fb52bd 100644
--- a/hs/src/Aoc/Y2020.hs
+++ b/hs/src/Aoc/Y2020.hs
@@ -22,6 +22,7 @@ import qualified Aoc.Y2020.D16 as D16
 import qualified Aoc.Y2020.D17 as D17
 import qualified Aoc.Y2020.D18 as D18
 import qualified Aoc.Y2020.D19 as D19
+import qualified Aoc.Y2020.D20 as D20
 
 year :: Year
 year = Year 2020
@@ -44,4 +45,5 @@ year = Year 2020
   , (17, D17.day)
   , (18, D18.day)
   , (19, D19.day)
+  , (20, D20.day)
   ]
diff --git a/hs/src/Aoc/Y2020/D20.hs b/hs/src/Aoc/Y2020/D20.hs
new file mode 100644
index 0000000..e78a98a
--- /dev/null
+++ b/hs/src/Aoc/Y2020/D20.hs
@@ -0,0 +1,120 @@
+{-# LANGUAGE OverloadedStrings #-}
+
+module Aoc.Y2020.D20
+  ( day
+  ) where
+
+import           Data.List
+import           Data.Maybe
+import           Control.Monad
+
+import qualified Data.Map  as Map
+import qualified Data.Set  as Set
+
+import           Aoc.Day
+import           Aoc.Parse
+
+newtype Tile = Tile [[Bool]] -- List of rows
+  deriving (Show)
+
+tLeft :: Tile -> [Bool]
+tLeft (Tile l) = map head l
+
+tRight :: Tile -> [Bool]
+tRight (Tile l) = map last l
+
+tTop :: Tile -> [Bool]
+tTop (Tile l) = head l
+
+tBottom :: Tile -> [Bool]
+tBottom (Tile l) = last l
+
+tTranspose :: Tile -> Tile
+tTranspose (Tile l) = Tile $ transpose l
+
+tFlipV :: Tile -> Tile
+tFlipV (Tile l) = Tile $ reverse l
+
+tFlipH :: Tile -> Tile
+tFlipH (Tile l) = Tile $ map reverse l
+
+tTurnCw :: Tile -> Tile
+tTurnCw = tFlipH . tTranspose
+
+tTurnCcw :: Tile -> Tile
+tTurnCcw = tFlipV . tTranspose
+
+tRotations :: Tile -> [Tile]
+tRotations = take 4 . iterate tTurnCw
+
+tVariations :: Tile -> [Tile]
+tVariations t = tRotations t ++ tRotations (tFlipH t)
+
+parser :: Parser (Map.Map Int Tile)
+parser = Map.fromList <$> (tile `sepBy` newline)
+  where
+    field = (False <$ string ".") <|> (True <$ "#")
+    row = sequenceA (replicate 10 field) <* newline
+    tile = do
+      tid <- string "Tile " *> decimal <* string ":" <* newline
+      rows <- sequenceA $ replicate 10 row
+      pure (tid, Tile rows)
+
+deduplicate :: (Ord a) => [a] -> [a]
+deduplicate = Set.toList . Set.fromList
+
+type Pos = (Int, Int)
+
+vicinity :: Pos -> [Pos]
+vicinity (x, y) = [(x - 1, y), (x + 1, y), (x, y - 1), (x, y + 1)]
+
+freeAdjacents :: Map.Map Pos a -> [Pos]
+freeAdjacents m =
+  let adjacents = deduplicate $ concatMap vicinity $ Map.keys m
+      taken = Map.keysSet m
+  in  filter (not . (`Set.member` taken)) adjacents
+
+unplaced :: Map.Map Int Tile -> Map.Map Pos (Int, Tile) -> [Int]
+unplaced tiles placed = Set.toList $ Map.keysSet tiles Set.\\ Set.fromList (map fst $ Map.elems placed)
+
+valid :: Map.Map Pos (Int, Tile) -> Pos -> Tile -> Bool
+valid placed (x, y) tile = validLeft && validRight && validTop && validBottom
+  where
+    validAt pos f g = maybe True (\a -> f (snd a) == g tile) $ placed Map.!? pos
+    validLeft   = validAt (x - 1, y) tRight  tLeft
+    validRight  = validAt (x + 1, y) tLeft   tRight
+    validTop    = validAt (x, y - 1) tBottom tTop
+    validBottom = validAt (x, y + 1) tTop    tBottom
+
+place :: Map.Map Int Tile -> Map.Map Pos (Int, Tile) -> Maybe (Map.Map Pos (Int, Tile))
+place tiles placed = listToMaybe $ do
+  tid <- unplaced tiles placed
+  let tile = tiles Map.! tid
+  pos <- freeAdjacents placed
+  var <- tVariations tile
+  guard $ valid placed pos var
+  pure $ Map.insert pos (tid, var) placed
+
+whileJust :: (a -> Maybe a) -> a -> a
+whileJust f a = maybe a (whileJust f) $ f a
+
+placeAll :: Map.Map Int Tile -> Map.Map Pos (Int, Tile)
+placeAll tiles = whileJust (place tiles) $ Map.singleton (0, 0) $ head $ Map.assocs tiles
+
+corners :: [Pos] -> [Pos]
+corners positions =
+  let minX = minimum $ map fst positions
+      maxX = maximum $ map fst positions
+      minY = minimum $ map snd positions
+      maxY = maximum $ map snd positions
+  in  (,) <$> [minX, maxX] <*> [minY, maxY]
+
+solver :: Map.Map Int Tile -> IO ()
+solver tiles = do
+  putStrLn ">> Part 1"
+  let placed = placeAll tiles
+      cornerIds = map (fst . (placed Map.!)) $ corners $ Map.keys placed
+  print $ product cornerIds
+
+day :: Day
+day = dayParse parser solver