open System
let p = stdin.ReadLine().Split()|>Array.map int
let a=p.[0]
let b=p.[1]
[<EntryPoint>]
let main argv =
    match (a+b)%2=0 with
    |true -> printfn "%d" ((a+b)/2)
    |false -> printfn "%s" "IMPOSSIBLE"
    0
