Problem:
Score: 100 points

Problem Statement

We have two distinct integers A and B.

Print the integer K such that |A - K| = |B - K|.

If such an integer does not exist, print IMPOSSIBLE instead.

Constraints

All values in input are integers.

0 \leq A,\ B \leq 10^9

A and B are distinct.

Input

Input is given from Standard Input in the following format:

A B

Output

Print the integer K satisfying the condition.

If such an integer does not exist, print IMPOSSIBLE instead.

Sample Input 1

2 16

Sample Output 1

9

|2 - 9| = 7 and |16 - 9| = 7, so 9 satisfies the condition.

Sample Input 2

0 3

Sample Output 2

IMPOSSIBLE

No integer satisfies the condition.

Sample Input 3

998244353 99824435

Sample Output 3

549034394

Solution in Kotlin (1.0.0) (judge verdict: Accepted)

fun main(args: Array<String>) {
val read = readLine()!!.reader().readText().split(" ")
    val a = read[0].toInt()
    val b = read[1].toInt()
    val max = if (a < b) b else a

    if (a != b) {
        repeat(max) {
            val x = Math.abs(a - it)
            val y = Math.abs(b - it)
            if (x == y) {
                println(it)
                return
            }
        }
        println("IMPOSSIBLE")
    } else {
        println("IMPOSSIBLE")
    }
}