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 extends App {
  var a = readLine.split(" ").map(_.toInt)
  var b = 0
  var sum = 0
  for(i <- 0 to a.max) {
    if(Math.abs(a(0)-i) == Math.abs(a(1)-i)) {
      b = 1
      sum = i
    }
  }
  print(if(b != 1) {"IMPOSSIBLE"} else {sum})
}