import qualified Data.Map as M
main = do
  xs <- fmap (tail . lines) getContents
  let m = M.fromListWith (+) $ zip xs (repeat 1)
      (a,b) = M.partitionWithKey (\k _ -> reverse k == k) m
      (c,d) = M.partitionWithKey (\k _ -> reverse k > k) b
      s = concatMap (\(k,x) -> concat $ replicate x k) $ M.toAscList
        $ M.union (M.map (`div`2) a) (M.intersectionWith min c (M.mapKeys reverse d))
  putStrLn $ s ++ concat (take 1 [k | (k,x) <- M.toAscList a, odd x]) ++ reverse s

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NgpmZGsKam52CnZuagprZGYKcWFxCmJoaA==

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