fun main (args : Array<String>) {
	val (a, b) = readLine()!!.split(" ")
	val A : Double = a.toDouble()
	val B : Double = b.toDouble()
	var result : Double = (A + B)
	return if( (result % 2) == 0.0 ) {
		println("${result.toInt()/2}")
	} else {
		println("IMPOSSIBLE")
	}	
}