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 Java8 (OpenJDK 1.8.0) (judge verdict: Wrong Answer)

import java.util.Scanner;

public class Main{
        public  static void main(String[] args){
                calc();
        }
        static int calc(){
                Scanner sc = new Scanner(System.in);
                int a=sc.nextInt();
                int b=sc.nextInt();
                int c,d;
                if((a+b)%2==0){
                        c=(a+b)/2;
                        if(c>0){
                        return c;
                        }else{
                                d =c*(-1);
                                return d;
                        }
                }else{
                        System.out.println("IMPOSSIBLE");
                }
                return 0;
        }
}