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.util.*;

public class Main {
	Scanner sc;
	int a,b;
	
	static String testInput = "998244353 99824435";
/*-------------
 * constructor
 */
	public Main() {
		sc = new Scanner(System.in);
	}
	public Main(String input) {
		sc = new Scanner(input);
	}
	
/*------------------
 * instance methods
 */
	void out(Object o) {
		System.out.println(o);
		System.out.flush();
	}
	
	public void exec() throws Exception {
		a = sc.nextInt();
		b = sc.nextInt();
		
		int k = (a+b)/2;
		if (k*2 != a+b) out("IMPOSSIBLE");
		else out(k);
	}
	
/*------
 * main
 */
	public static void main(String[] args) throws Exception {
		Main m = new Main();
		m.exec();
	}
}