import java.util.*
 
fun main(args: Array<String>) {
  
  var line = readLine()!!.split(" ")!!
  var a = line[0].toInt()!!
  var b = line[1].toInt()!!

  if ((a+b)%2 == 0){
    println((a+b)/2)
  }else {
    println("IMPOSSIBLE")
  }
 
}