import Data.List

main = do
  n <- readLn :: IO Int
  ws <- (map read . words) <$> getLine
  let e = (sum ws) `div` 2
  let ws' = filter (<= e) ws
  putStrLn $ if (odd $ sum ws) || (sum ws' < e) then "impossible" else resolve e ws'
  
resolve e ws = resolve' (getPairs e 0 (reverse $ sort ws))
  where
    resolve' :: [(Int, [Int])] -> String
    resolve' [] = "impossible"
    resolve' pairs@((a,bs):ps)
      | a == e = "possible"
      | null bs = resolve' ps
      | otherwise = resolve' ((getPairs e a bs) ++ ps)

getPairs :: Int -> Int -> [Int] -> [(Int,[Int])]
getPairs e c (p:ps)
  | null ps = [(m, [])]
  | otherwise = (m, filter (<= (e - m)) ps):(getPairs e c (p:(tail ps)))
    where m = c + p