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 PHP (5.6.30) (judge verdict: Accepted)

<?php
  
while ($line = fgets(STDIN)) {
   $stdin[] = trim($line);
}
#$v2 = str_split($stdin[0]);
$v = explode(" ", $stdin[0]);


if($v[0] % 2 == 0 && $v[1] % 2 == 0){
  $hidari = $v[0] / 2;
  $migi = $v[1] / 2;
  $ans = $hidari + $migi;
}elseif($v[0] % 2 == 1 && $v[1] % 2 == 1){
  $hidari = $v[0] / 2;
  $migi = $v[1] / 2;
  $ans = $hidari + $migi;
}else{
  $ans = "IMPOSSIBLE";
}






echo $ans . PHP_EOL;