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: Time Limit Exceeded)

using System;
using System.Collections.Generic;



class Program
{

    static void Main(string[] args)
    {
        
        string[] arr = Console.ReadLine().Split(' ');
        int a = Math.Abs(int.Parse(arr[0]));
        int b = Math.Abs(int.Parse(arr[1]));
        

        for (int i =Math.Max(a,b); i > Math.Min(a, b); i--)
        {
            if (Math.Abs(a - i) == Math.Abs(b - i))
            {
                Console.WriteLine(i);
                return;
            }
            
        }
        Console.WriteLine("IMPOSSIBLE");

    }
}