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 C# (Mono 4.6.2.0) (judge verdict: Accepted)

using System;
using System.Collections.Generic;

public class CSharp{
    public static void Main(){
        Kyopuro chokudai = new Kyopuro();
	    chokudai.AtCoder();
	}
}

public class Kyopuro{

	int a,b;
	
    public void AtCoder(){
        string[] input;
		input = Console.ReadLine().Split(' ');
		a = int.Parse(input[0]);
		b = int.Parse(input[1]);
      
        int c = a + b;
      	if (c % 2 == 0) Console.WriteLine(c / 2);
      	else Console.WriteLine("IMPOSSIBLE");
		
		
    }
    public static void DebugWL(object x){ System.Diagnostics.Debug.WriteLine(x); }
}