open System

[<EntryPoint>]
let main argv =
  let xs = Console.ReadLine().Split(' ') |> Array.map int
  let ans =
    let A = xs.[0] in
    let B = xs.[1] in
    if (A+B)%2 = 0 then string ((A+B)/2) else "IMPOSSIBLE"
  ans |> Console.WriteLine
  0
