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

# スペース区切りの整数の入力
fscanf(STDIN, "%d %d", $a, $b);

$k = ($a + $b)/2;

if(is_int($k) === false){
    echo "IMPOSSIBLE", PHP_EOL ;
}else{
    echo "$k", PHP_EOL ;
}