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(input){
  input = input.split(" ");
  input[0]=parseInt(input[0])
  input[1]=parseInt(input[1])

  if(input[0]%2==0 && input[1]%2==1){    
    console.log("IMPOSSIBLE")
    return 
  }
  if(input[0]%2==1 && input[1]%2==0){    
    console.log("IMPOSSIBLE")
    return 
  }
  if(input[0]>input[1]){
    console.log(input[1]+(input[0]-input[1])/2)
    return 
  }
  if(input[1]>input[0]){
    console.log(input[0]+(input[1]-input[0])/2)
    return 
  }
}

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