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 Scala (2.11.7) (judge verdict: Accepted)

object Main {

  def main(args: Array[String]): Unit = {
    val sc = new java.util.Scanner(System.in)

    val A, B = sc.nextLong()
    val K1 = (A + B) / 2
    val K2 = (- A - B) / 2

    val r = if (A == B) "0"
    else if ((A + B) % 2 == 0) {
      if (Math.abs(A - K1) == Math.abs(B - K1))
        K1.toString
      else if (Math.abs(A - K2) == Math.abs(B - K2))
        K2.toString
      else "IMPOSSIBLE"
    } else "IMPOSSIBLE"
    println(r)
  }

}