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: Wrong Answer)

using System;
using System.Collections.Generic;
using System.Linq;
using System.IO;



namespace AtCodeeeer
{
    class AtCOOOOOOOOOOOder
    {
        static void Main(string[] args)
        {
            //string A = Console.ReadLine();//Aから読み込む
            string[] s = Console.ReadLine().Split(' ');//　文字たちを読み込む
            int A = int.Parse(s[0]);//
            int B = int.Parse(s[1]);//
            int K = 0 ;
            
            //string s = Console.ReadLine();//こいつだけは文字列
            //ここまでが入力部分でーす💛
            if(System.Math.Abs(A-K) == System.Math.Abs(B-K)){
           Console.WriteLine(K);
            }else{
           Console.WriteLine("IMPOSSIBLE");     
            }
        }
    }
}