import Data.List

main = do
  getLine
  l <- getLine
  let ws = map read $ words l
  let r = compute ws
  putStrLn $ if r then "possible" else "impossible"

compute ws = even total && elemO (div total 2) (make $ sort ws) where
  total = sum ws

make [] = [0]
make (x:xs) = merge ws $ map (x+) ws where ws = make xs

merge xs [] = xs
merge [] ys = ys
merge xxs@(x:xs) yys@(y:ys)
 | x < y = x : merge xs yys
 | x > y = y : merge xxs ys
 | True  = x : merge xs ys

elemO v xs = (v ==) $ head $ dropWhile (v >) xs