let reads f = stdin.ReadLine().Split(' ') |> Array.map f
let (|Even|Odd|) i = if i % 2 = 0 then Even else Odd

reads int
|> (fun xs -> xs.[0] + xs.[1])
|> (fun x -> match x with Even -> string (x / 2) | Odd -> "IMPOSSIBLE")
|> printfn "%s"