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)

fun main(args: Array<String>) {
//    val mod = 1000000007L
    val mod = 2019
//    val N = readLine()!!.toInt()
    val (A,B) = readLine()!!.split(" ").map(String::toLong)
    var ans = "IMMPOSSIBLE"

    for (k in 0..Math.max(A,B)) {
        if (Math.abs(A-k) != Math.abs(B-k)) continue
        ans = "$k"
        break
    }
    println(ans)
}