let () = Scanf.scanf "%d\n" @@ fun n ->
  let as_ = Array.init n @@ fun _ -> Scanf.scanf "%d " @@ fun a -> a in
  let d = Array.fold_left max min_int as_ in
  let count_as = Array.make n 0 in
  Array.iter (fun a -> count_as.(a) <- 1 + count_as.(a)) as_;
  print_endline @@
  if
    Array.fold_right (fun n -> ( && ) (n = 0))
      (Array.sub count_as 0 ((d + 1) / 2)) true &&
    count_as.((d + 1) / 2) = 1 + d mod 2 &&
    Array.fold_right (fun n -> ( && ) (1 < n))
      (Array.sub count_as ((d + 1) / 2 + 1) (d / 2)) true
  then "Possible"
  else "Impossible"
