let harmony a b =
  let harmony_hojo a b =
    if (a + b) mod 2 = 0 then Some ((a + b) / 2)
                         else None
  in
    match harmony_hojo a b with
        None -> "IMPOSSIBLE"
      | Some (p) -> string_of_int p

let a, b = Scanf.sscanf (read_line ()) "%d %d" (fun a b -> (a, b))
let () = Printf.printf "%s\n" (harmony a b)