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: Wrong Answer)

import java.lang.Math.abs
import java.util.*

fun main(args: Array<String>) {
    try {
        val sc = Scanner(System.`in`)
        val a = sc.nextInt()
        val b = sc.nextInt()
        val kk = abs(a) + abs(b)
        if (kk % 2 != 0) {
            println("IMPOSSIBLE")
        }
        val k = kk / 2
        println(k)
    } catch(e: Exception) {
        println(e)
    }
}