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: Runtime Error)

process.stdin.resume()
process.stdin.setEncoding('utf8')

let input_string = "";
let reader = require('readline').createInterface({
    input: process.stdin,
    output: process.stdout
})

reader.on('line', (line) => {
    input_string = line;
})

let numArray = input_string.split(' ')
let numA = Math.abs(numArray[0])
let numB = Math.abs(numArray[1])
let addNum = numA - numB

if(addNum%2 == 0){
  reader.on('close', () => {
      console.log(addNum/2)
  })
}else{
  reader.on('close', () => {
      console.log('IMPOSSIBLE')
  })
}