Problem A

A.hs

import Control.Monad
import Data.List (find, unfoldr)

main = interact (unlines . map (uncurry fmt) . zip [1..] . map solve . parse)
    where fmt n (Just (x, y)) = "Case #" ++ show n ++ ": " ++ unwords [show x, show y]
          fmt _ Nothing = ""

solve (c, xs) = go $ zip [1..] xs
    where go ((i, n):ys) =
            case find (\(_, n') -> n' + n == c) ys of
                Just (i', _) -> Just (i, i')
                Nothing      -> go ys
          go [] = Nothing

parse = unfoldr g . tail . lines
    where g (a:_:b:xs) = return ((read a, map read $ words b), xs)
          g _          = mzero