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)

import java.util.*

fun main(vararg args: String) {
    val scanner = Scanner(System.`in`)
    val a = scanner.nextInt()
    val b = scanner.nextInt()
    val diff = Math.abs(a - b)
    if (diff % 2 == 1) {
        println("IMPOSSIBLE")
        return
    }
    val k = Math.min(a, b) + diff / 2
    println(k)
}