object Main {
  def main(args: Array[String]): Unit = {
    val sc = new java.util.Scanner(System.in)
    val ab = List.fill(2)(sc.nextInt)
    sc.close()

    val c = (ab.max - ab.min) / 2
    val ac = Math.abs(ab.max - c)
    val bc = Math.abs(-c - ab.min)
    ac == bc match {
      case true => println(ac)
      case _    => println("IMPOSSIBLE")
    }
  }
}