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: Wrong Answer)

function Main(input) {
  input = input.split("\n");
  const n = parseInt(input[0].split(" ")[0], 10);
  const P = input[1].split(" ");

  const PSorted = P.concat().sort();

  var count = 0;

  for (var i = 0; i < n; i++) {
    P[i] !== PSorted[i] && count++;
  }

  console.log(2 >= count ? 'YES' : 'NO');
}

// *出力
// number
// console.log('%d', );
// string
// console.log('%s', );

Main(require("fs").readFileSync("/dev/stdin", "utf8"));