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;

public class AtCoderA
{
    static void Main()
    {
        long a;
        long b;
        {
            var line = Console.ReadLine();
            var words = line.Split(' ');
            a = long.Parse(words[0]);
            b = long.Parse(words[1]);
        }

        for (long k = 0; k < Math.Max(a, b); k++)
        {
            if (Math.Abs(a - k) == Math.Abs(b - k))
            {
                Console.WriteLine(k);
            }
        }

        Console.WriteLine("IMPOSSIBLE");
    }
}