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.concurrent.atomic.AtomicBoolean;
import java.util.stream.IntStream;

public class Main
{
    public static void main(String[] args) {
        int a = Integer.parseInt(args[0]);
        int b = Integer.parseInt(args[1]);
        AtomicBoolean ab = new AtomicBoolean(false);
        IntStream.range(0, (int)Math.pow(10, 9)).forEach(k -> {
            int absA = Math.abs(a - k);
            int absB = Math.abs(b - k);
            if (absA == absB) {
                ab.set(true);
                System.out.println(k);
                return;
            } 
        });
        if (!ab.get()) {
            System.out.println("IMPOSSIBLE");
        }
    }
}