import java.util.Scanner

object Main {
  def main(args: Array[String]): Unit = {
    //nextLineとnextを同時に使ったら死ぬ
    val sc = new Scanner(System.in)
    val a, b = sc.nextLong()
    if (a + b % 2 == 0) {
      println((a + b) / 2)
    } else {
      println("IMPOSSIBLE")
    }
  }
}
