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 12.16.1) (judge verdict: Accepted)

const main = stdin => {
  const input = stdin.trim().split('\n')

  const [A, B] = input[0].split(' ').map(x => parseInt(x))
  if (A % 2 !== B % 2) {
    console.log('IMPOSSIBLE')
    return
  }

  console.log((A + B) / 2)
}

// 標準入力
main(require("fs").readFileSync("/dev/stdin", "utf8"))