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: Runtime Error)

import java.util.*;

class Main{
	
	public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);
        int cnt = 0;
        int num = sc.nextInt();
        List<Integer> alist = new ArrayList<Integer>();
        for(int i = 0; i < num;i++) {
        	alist.add(sc.nextInt());
        }
        
	List<Integer> list = new ArrayList<Integer>();
	for(int i = 0;i < num;i++) {
		list.add(i);
	}
	for(int a : alist){
        for(int b : list){
            if(a==b){
                cnt+=1;
                break;
            }
       
        }
    }
	if(num - cnt <= 2 ) {
		System.out.println("YES");
	}else {
		System.out.println("NO");
	}
	
	}
}