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 JavaScript (node.js v5.12) (judge verdict: Accepted)

function main(arg) {
    const A = parseInt( arg.split(/\s/)[0] );
    const B = parseInt( arg.split(/\s/)[1] );
    var ans = (A+B)/2;
    if(ans==parseInt(ans)){
        console.log(ans);
    }else{
        console.log("IMPOSSIBLE");
    }
}
main(require('fs').readFileSync('/dev/stdin', 'utf8'));