import Data.List
main :: IO ()
main = mapM_ (print . find4A) ["1 5 3 6 : 10", "1 5 3 6 7 : 10"]
find4A :: String -> [String]
find4A s = (flip (map . (!!)) =<< elemIndices y . map expr) $ combo [x1] opPlusDigitList
  where
    combo as []     = as
    combo as (h:t)  = combo [a ++ b | a <- as, b <- h] t
    opPlusDigitList = map (\x -> [op ++ x | op <- ops]) $ xs
    (xs0, y0)       = break (==':') s
    x1:xs           = words xs0
    y               = read $ tail y0         :: Double
    ops             = [" + ", " - ", " * ", " / "]
expr :: String -> Double
expr s = go (map read xxs) yys
  where
    (xxs, yys) = partition (`notElem` ["+", "-", "*", "/"]) $ words s
    go [x1]       _        = x1
    go (x1:x2:xs) ("*":ys) = go ((x1 * x2) : xs) ys
    go (x1:x2:xs) ("/":ys) = go ((x1 / x2) : xs) ys
    go (x1:xs)    ("+":ys) = x1 + go xs ys
    go (x1:x2:xs) ("-":ys) = x1 + go ((-x2) : xs) ys
    go _          _        = -1