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 Atcoder
{
    class ABC135
    {
        static void Main(string[] args)
        {
            string[] str = Console.ReadLine().Split(' ');
            int a = int.Parse(str[0]);
            int b = int.Parse(str[1]);
            if( (a + b)%2 == 0)
            {
                Console.Write((a + b) / 2);
            }
            else
            {
                Console.Write("IMPOSSIBLE");
            }
        }
    }
}