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;

namespace ABC135
{
    class Program
    {
        static int Main(string[] args)
        {
            problemA();
            return 0;
        }

        static void problemA()
        {
            string[] input = Console.ReadLine().Split(' ');

            int A = int.Parse(input[0]);
            int B = int.Parse(input[1]);

            int C = Math.Abs(A - B);

            if (C % 2 != 0)
            {
                Console.WriteLine("IMPOSSIBLE");
            }
            else
            {
                if(A > B)
                {
                    Console.WriteLine(A - C/2);
                }
                else if (B > A)
                {
                    Console.WriteLine(B - C/2);
                }
            }

        }
    }
}