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
    2 <= count_as.(d) &&
    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_left ( && ) true
      (Array.init (d / 2)
        (fun i -> count_as.((d + 1) / 2 + i) <= count_as.((d + 1) / 2 + i + 1)))
  then "Possible"
  else "Impossible"
