-- yukicoder My Practice
-- author: Leonardone @ NEETSDKASU
-- http://yukicoder.me/problems/19

import Data.Bits

main = do
    n <- readLn
    w <- getLine
    putStrLn $ main' n w

main' n ws =
    let
        w = map read $ words ws
        s = foldl1 (+) w `div` 2
        a = zip [1..n] w
        x = shiftL (1::Int) n
    in
        solve s (x - 1) a

solve _ 0 _ = "impossible"
solve n x a =
    if n == foldl calc x a
        then "possible"
        else solve n (x - 1) a
        
calc x (i,e) = if testBit i x then e else 0