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.Arrays;
import java.util.List;
import java.util.Scanner;
import java.util.stream.Collectors;

public class Main {

	public static void main(String[] args) {
		Scanner scanner = new Scanner(System.in);

		int a = 0, b = 0;

		try {
			String[] s = scanner.nextLine().split(" ");
			a = Integer.parseInt(s[0]);
			b = Integer.parseInt(s[1]);
		} catch (NumberFormatException e) {
		} catch (NullPointerException e) {
		}

		scanner.close();
		
		if ((a - b % 2) == 0) {
			System.out.println("IMPOSSIBLE");
		} else {
		 	int v = Math.abs (a - b) / 2;
		 	
		 	int ret;
		 	if (a > b) {
		 		ret = b + v;
		 	} else {
		 		ret = a + v;
		 	}
		 	
		 	System.out.println(ret);
		 	
		}
	}
}