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diff --git a/2024/13-Claw_Contraption/first.hs b/2024/13-Claw_Contraption/first.hs
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+++ b/2024/13-Claw_Contraption/first.hs
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+-- requires cabal install --lib megaparsec parser-combinators heap vector
+module Main (main) where
+
+import           Control.Monad        (void, when)
+import           Data.Either
+import qualified Data.Matrix          as MTX
+import           Data.Ratio
+import           Data.Void            (Void)
+import           Text.Megaparsec
+import           Text.Megaparsec.Char
+
+exampleExpectedOutput = 480
+
+type Pair = (Rational, Rational) -- X Y
+type Entry = (Pair, Pair, Pair) -- ButtonA ButtonB Prize
+type Input = [Entry]
+
+type Parser = Parsec Void String
+
+parseNumber :: Parser Rational
+parseNumber = fromInteger . read <$> some digitChar
+
+parseButton :: Parser Pair
+parseButton = (,) <$> (string "Button " *> letterChar *> string ": X+" *> parseNumber)
+                  <*> (string ", Y+" *> parseNumber <* eol)
+
+parsePrize :: Parser Pair
+parsePrize = (,) <$> (string "Prize: X=" *> parseNumber)
+                 <*> (string ", Y=" *> parseNumber <* eol)
+
+parseEntry :: Parser Entry
+parseEntry = (,,) <$> parseButton
+                  <*> parseButton
+                  <*> parsePrize
+
+parseInput' :: Parser Input
+parseInput' = some (parseEntry <* optional eol) <* eof
+
+parseInput :: String -> IO Input
+parseInput filename = do
+  input <- readFile filename
+  case runParser parseInput' filename input of
+    Left bundle  -> error $ errorBundlePretty bundle
+    Right input' -> return input'
+
+-- ax + bx = c
+-- ay + by = d
+compute :: Input -> Rational
+compute = sum . map compute'
+  where
+    compute' ((ax, ay), (bx, by), (px, py)) = let (Right sol) = MTX.rref $ MTX.fromList 2 3 [ ax, bx, px
+                                                                                            , ay, by, py ]
+                                                  x = sol MTX.! (1, 3)
+                                                  y = sol MTX.! (2, 3)
+                                              in if (denominator x == 1 && denominator y == 1) then 3 * x + y else 0
+
+main :: IO ()
+main = do
+  example <- parseInput "example"
+  let exampleOutput = compute example
+  when  (exampleOutput /= exampleExpectedOutput)  (error $ "example failed: got " ++ show exampleOutput ++ " instead of " ++ show exampleExpectedOutput)
+  input <- parseInput "input"
+  print $ compute input