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: Runtime Error)

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

  const N = parseInt(inputs[0])
  const sequence = inputs[1].split(' ').map(x => parseInt(x))

  const sortedSample = sequence.slice().sort((a, b) => a - b)

  let diff = 0
  for (let i = 0; i < N; i++) {
    //    console.log(sequence[i], sortedSample[i]);
    if (sequence[i] !== sortedSample[i]) {
      diff++
    }
  }

  diff === 2 || diff === 0 ? console.log('YES') : console.log('NO')

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