


object Main extends App {

  val Array(a,b) = scala.io.StdIn.readLine.split(" ").map(_.toLong)

  def solve(): String = {
    val ans: Double = (a * a - b * b).toDouble / (2 * (a - b))
    if(ans % 1 == 0) ans.toInt.toString else "IMPOSSIBLE"
  }
  println(solve)
}
