fun ans(a: Int, b: Int): Int {
    return (a + b) / 2
}

fun main(args: Array<String>) {
    val n = readLine()!!.split(" ")
    val a = n[0].toInt()
    val b = n[1].toInt()
    if ((a + b) % 2 == 1) {
        println("IMPOSSIBLE")
    } else {
        println(ans(a, b))
    }
}
