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.*;
import java.io.*;

class Main {

	void solve () {

		int a = in.nextInt(), b = in.nextInt();
		
		if (Math.abs(a-b)%2 == 1) out.println("IMPOSSIBLE");
		else {
			int dif = Math.abs(a-b)/2;
			out.println(Math.max(a,b)-dif);
		}
		
	}

	public static PrintWriter out = new PrintWriter(System.out);
	public static Scanner in = new Scanner(System.in);
	public static void main(String[] args) {
		new Main().solve();
		out.flush();
	}

}