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 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))
    }
}