// Learn more about F# at http://fsharp.org

open System

[<EntryPoint>]
let main _argv =
  
    let ab = stdin.ReadLine().Split(' ') |> Array.map int
    let a = ab.[0]
    let b = ab.[1]
    
    if (a + b) % 2  = 0 then
        printfn "IMPOSSIBLE"
    else
        printfn "%d" ((a + b) / 2)
    0 // return an integer exit code
