[<AutoOpen>]
module Program

open System

let read f = stdin.ReadLine() |> f
let reads f = stdin.ReadLine().Split() |> Array.map f

[<EntryPoint>]
let main argv =
    let [|a;b|] = reads int
    let (x, y) = (min a b, max a b)
    let z = (y - x) / 2 + x
    if (y - x) % 2 = 0
    then printfn "%i" z
    else printfn "IMPOSSIBLE"
    0