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)

package atcoder.abc135;

import java.util.Scanner;

public class Main
{
    public static void main( String[] args) {
        try (Scanner s = new Scanner(System.in)) {
            int a = s.nextInt();
            int b = s.nextInt();
            int max = (int) Math.pow(10, 9);
            boolean found = false;
            for (int i = 0; i < max; i++) {
                if (Math.abs(a - i) == Math.abs(b - i)) {
                    System.out.println(i);
                    found = true;
                }
            }
            if (!found) {
                System.out.println("IMPOSSIBLE");
            }
        } catch (Exception e) {
            e.printStackTrace();
        }
    }
}