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: Accepted)

import java.io.PrintStream;
import java.util.Scanner;

public class Main {
	public static void main(String[] args) {
		Scanner sc = null;
		PrintStream out = null;
		try {
			sc = new Scanner(System.in);
			out = System.out;
			exec(sc, out);
		} catch (Exception e) {
			e.printStackTrace();
			System.exit(1);
		} finally {
			if(out != null) {
				out.close();
			}
			if(sc != null) {
				sc.close();
			}
		}
		System.exit(0);
	}
	public static void exec(final Scanner sc, final PrintStream out) throws Exception{
		final int A = sc.nextInt();
		final int B = sc.nextInt();
		final int C = A+B;
		if(C%2 == 1) {
			out.println("IMPOSSIBLE");
		}else {
			out.println(C/2);
		}
	}
}