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;
class Program
{
    //static List<string> str = new List<string>();
	static void Main(string[] args)
	{
		// 入力フェーズ
		string[] input = Console.ReadLine().Split(' ');
		int A = int.Parse(input[0]);
		int B = int.Parse(input[1]);

    int K2 = A+B;
		if(K2%2 == 0)
			Console.WriteLine("" + (K2/2));
		else
		  Console.WriteLine("IMPOSSIBLE");

    }

}